The W.H. Benson collections are mainly held in UMZC, Cambridge, and most are considered the primary type specimens. However, Benson’s type specimens were not very explicit, and difficult to specify due to mislaid labels and subsequent substitution of the original labels (see Naggs 1997 and Preece et al. 2022 for more detail). However, in the original description of R. haughtoni, Benson stated that he had received several specimens, and one was alive. The W.H. Benson ex. R. MacAndrews collection contains one lot of two specimens. One specimen with a mummified soft body remaining inside the shell corresponds with Benson’s statement (Benson 1860: 96). This specimen lot was designated as the lectotype UMZC I.103445.A (Fig. 11A) by Preece et al. (2022: 78), and another shell, UMZC I.103445.B (Fig. 11 B), becomes the paralectotype.

Myanmar: Damaltha, Burma: NHMUK 88.12.4.1985–1986 (specimen with “x” was figured in Gude 1921, fig. 22; Fig. 11C), SMF 130508. Moulmein, Tenasserim, Burma: NHMUK 75.06.5.5 (4 shells), NHMUK 1903.7.1.1566 (2 shells), NHMUK ex. E.R. Sykes coll. Acc. 1825 (1 shell), NHMUK ex. Cuming coll. (1 shell), NHMUK ex. Oldham coll. (1 shell). Damatha, Burma: NHMUK ex. McAndrew coll. (4 shells). Dhammatat Cave, Mawlamyine District, Mon State: CUMZ 10048 (Figs 9A, 11D–F, 16A), CUMZ 10049.

Figure 9. 

Live snails of the species group I A Rhiostoma haughtoni specimen CUMZ 10048/2 from Dhammathat Cave, Myanmar B–D Rhiostoma samuiense B specimen CUMZ 3989 from Samui Island, Surat Thani C specimen CUMZ 3996 from Perlis, Malaysia (topotype of Rhiostoma chupingense) D specimen CUMZ 4713 of “proboscis” morphotype from Surat Thani E, F copulation pair of Rhiostoma samuiense specimen CUMZ 4713 from Surat Thani shows unsuccessfully transferred spermatophore (female on the right in E and on the left in F), and the red arrows indicate a spermatophore illustrated in Fig. 7D. All figures are not to scale.

Shell thick and depressed. Detached whorl short, curved and slightly descending. Breathing device notch-shaped. Shell colour uniformly brownish or with dark zigzag pattern.

In addition, the shell morphology of R. haughtoni differs from R. housei in having a notch-shaped breathing device, and a short detached whorl, while R. housei bears a tubular breathing device and the detached whorl is medium in length.

Shell. Shell small to medium, cW 16.9–20.4 mm, cH 8.7–10.9 mm, thickened, and sub-discoidal to discoidal shape; detached-whorl length 3.0–8.0 mm. Apex acute; spire slightly elevated. Whorls 5 to 6, convex, increasing regularly; suture wide and deep; last whorl rounded. Shell surface with fine growth lines. Periostracum thick or thin, corneous, and transparent. Shell colour uniformly brownish, sometimes with dark brown zigzag pattern; ventral shell surface paler in colour; with or without a dark brown spiral band on periphery. Detached whorl shorter than apertural width, curved and descending. Peristome circular and double; lip thickened, slightly expanded and multi-layered. Aperture opened sub-laterally. Breathing device notch-shaped; outer lip protruding, with narrow groove; inner lip with shallow to deep incision. Umbilicus widely open and deep. Operculum calcareous, tall cup-shaped, and multispiral (Fig. 11).

Radula. Taenioglossate radula arranged in inverted V-shaped rows. Central tooth with well-developed central cusp and two lateral cusps on each side; central cusp large with pointed tip; lateral cusps triangular and tapering in size. Lateral teeth consisting of four cusps; central cusp large, elongate, and flanked by pointed tip of two small inner cusps and one small outer cusp. Inner and outer marginal teeth each composed of three cusps; central cusp large and with dull tip, and flanked by pointed tip of one inner cusp and one outer cusp (Fig. 16A).

This species is currently known only from the type locality in Mon State, southern Myanmar (Fig. 10).

Figure 10. 

Distribution map of Rhiostoma haughtoni–species group.

Rhiostoma haughtoni is often regarded as a junior synonym of R. housei from central Thailand (Habe 1965; Abbott 1989). However, R. housei exhibits significant intra-specific variation in shell morphology across its range. Furthermore, recent allozyme studies have suggested that R. housei is likely to represent a complex of cryptic species (Prasankok et al. 2011). In this study, the topotypic specimens of R. haughtoni clearly show a significant genetic distance from R. housei, proving it to be clearly distinct from R. housei.

The shell of R. haughtoni can vary from monochrome brownish or whitish without a peripheral band (Fig. 11D) to brownish zigzag patterns or blotches and thin peripheral band (Fig. 11E, F). The length of the detached whorl and shape of breathing device are relatively conserved, at least in the topotype population.

Figure 11. 

Shell of Rhiostoma haughtoni A lectotype UMZC I.103445.A from Moulmein B paralectotype UMZC I.103445.B C specimen NHMUK 88.12.4.1985–1986 figured in Gude (1921, fig. 22) from Damaltha, Burma D–F specimens CUMZ 10048 from Dhammatat Cave, Mon, Myanmar.

Tomlin (1932) had obtained several living specimens, and the original description included an illustration and one set of shell measurements. There are two lots from the Melvill-Tomlin collection, which are considered the syntypes NMW 1955.158.01104 (1 shell; Fig. 12A) and NMW 1955.158.01105 (6 shells; Fig. 12B). Lectotype (designation in Sutcharit et al. (2019)) NHMUK 1938.10.25.1 (Fig. 13A) of Rhiostoma chupingense Tomlin, 1938, from Bukit Chuping, Perlis, Malaysia; paralectotypes NMW 1955.158.01101 (3 shells), NMW 2.1981.118.02703 (1 shell).

Figure 12. 

Shell of Rhiostoma samuiense A syntype NMW 1955.158.1104 from Kaw Samui, Siam B syntype NMW 1955.158.1105 form type locality C specimen NMW 2.1981.118.20704 Kaw Samui D specimen CUMZ 3989 from Koh Samui, Surat Thani E, F specimen from Koh Tan, Surat Thani E specimen CUMZ 4791 and F specimen CUMZ 4707.

Figure 13. 

Shell of Rhiostoma samuiense A lectotype of R. chupingense NHMUK 1938.10.25.1 from Bukit Chuping, Perlis, Malay State B specimen CUMZ 3996 from Bukit Chuping, Perlis, Malaysia (topotype) C specimen CUMZ 3986 from Bang Saphan, Prachuap Khiri Khan D specimen CUMZ 4708 from Khanom, Nakhon Si Thammarat E specimen CUMZ 3990 from Tha Chana, Surat Thani F specimen CUMZ 3997 from Sungai Jernih, Perlis, Malaysia.

Malaysia: Bukit Chuping, Kangar, Perlis: CUMZ 3844, 3996 (Figs 9C, 13B), 4746, 4768. Sungai Jernih Kangar, Kuala Perlis, Perlis: CUMZ 3850, 3997 (Fig. 13F), 4832. Myanmar: Lampane Cave, Ngawun, Tanintharyi: CUMZ 10214. Thailand: Kaw Samui: NMW 2.1981.118.20704 ex. T. Pain coll. (Fig. 12C). Khao Marong, Bang Sapan, Prachuap Khiri Khan: CUMZ 3865, 3874, 3986 (Fig. 13C), 4477, 4739, 4854, 4861, 4900, 10079, 10086, 10089. Ban Na Sarn, Surat Thani: CUMZ 4305, 4395. Khao Chongchang, Ban Na Sarn, Surat Thani: CUMZ 4306. Tham Kha Min, Ban Na Sarn, Surat Thani: CUMZ 4310. Wat Na Sarn, Ban Na Sarn, Surat Thani: CUMZ 3842. Donsak, Surat Thani: CUMZ 4308, 4309. Khao Chang, Donsak, Surat Thani: CUMZ 3866. Khao Pra, Donsak, Surat Thani: CUMZ 4777. Khao Sai, Donsak, Surat Thani: CUMZ 3992, 4759. Km8+500m, Donsak, Surat Thani: CUMZ 10094. Tham Khiriwong, Donsak, Surat Thani: CUMZ 3811, 10078. Wat Tham Po Ngam, Donsak, Surat Thani: CUMZ 3873. Kra Dae, Kanchanadit, Surat Thani: CUMZ 3869, 4790 (Fig. 14E). Tham Khuha, Kanchanadit, Surat Thani: CUMZ 4424, 4481. Wat Tham Petpanomwang, Kanchanadit, Surat Thani: CUMZ 3833, 3835, 3895, 3991, 4361, 4362, 4392, 4396, 4398, 4471, 4472, 4473, 4474, 4475, 4712 (Fig. 14A), 4713 (Figs 7C, 9D–F, 14B–D), 4721, 4741, 4782, 4785, 4793, 4802, 4812, 4875, 10065, 10066, 10067, 10069. Hin Lad Waterfall, Koh Samui, Surat Thani: CUMZ 3864, 4373, 4413, 4834, 4893. Koh Samui, Surat Thani: CUMZ 4307, 4332, 4333, 4334, 4335, 4397, 10073. Koh Tan, Koh Samui, Surat Thani: CUMZ 3863, 3890, 4382, 4415, 4432, 4707 (Fig. 12F), 4791 (Fig. 12E), 10070. Koh Wang Nok, Koh Samui, Surat Thani: CUMZ 3385, 3891. Na Muang Waterfall, Koh Samui, Surat Thani: CUMZ 4800, 3862, 3989 (Figs 9B, 12D, 16B), 4780. Wat Tham Yai, Tha Cha Na, Surat Thani: CUMZ 3990 (Fig. 13E), 4476, 4722, 4769, 4787, 4857, 4863, 4891, 4896, 10084, 10093, 10095. Wat Wichit Dittharam, Tha Cha Na, Surat Thani: CUMZ 10085. Wat Khao Hauy Hang, Hauy Yod, Trang: CUMZ 3840, 3846, 3945, 3993, 4774. Samet Chun Waterfall, Kanom, Nakhon Si Thammarat: CUMZ 4773. Tham Khao Krot, Kanom, Nakhon Si Thammarat: CUMZ 3944, 4000, 10087, 10092. Tham Wang Thong, Kanom, Nakhon Si Thammarat: CUMZ 3820, 4311, 4388, 4708 (Fig. 13D), 4781, 10091. Thongnian, Kanom, Nakhon Si Thammarat: CUMZ 10090. Wat Ao Sadet, Kanom, Nakhon Si Thammarat: CUMZ 10088. Tham Nam Wang Srithammasokkarach, Lan Saka, Nakhon Si Thammarat: CUMZ 4855. Wat Tham Khao Dang, Ron Phiboon, Nakhon Si Thammarat: CUMZ 10068. Wat Tham Khao Dang, Hintok, Ron Phiboon, Nakhon Si Thammarat: CUMZ 3838. Khao Chang (Sikeed), Sichon, Nakhon Si Thammarat: CUMZ 3894, 3999, 4786 (Fig. 14F). Khao Hu Thu, Khao Noi, Sichon, Nakhon Si Thammarat: CUMZ 3871. Khao Nam Lark (Sikeed), Sichon, Nakhon Si Thammarat: CUMZ 3893, 3898, 4757. Wat Tham Pra Chaison, Khao Chaison, Patthalung: CUMZ 10096. Khao Pu-Khao Ya, Si Banphot, Patthalung: CUMZ 3995, 4304, 4375, 4399, 4811, 10076, 10077. Khao Roop Chang, Padang Besa, Songkhla: CUMZ 3849, 4479, 10097. Tham Srikaesorn, Rattaphum, Songkhla: CUMZ 4488.

Figure 14. 

Shell of Rhiostoma samuiense of “proboscis” morphotype A specimen CUMZ 4712 from Kanchanadit, Surat Thani B–D specimens CUMZ 4713 from Kanchanadit, Surat Thani E specimen CUMZ 4790 from Kanchanadit, Surat Thani F specimen CUMZ 4786 from Sichon, Nakhon Si Thammarat.

Shell thick and nearly flattened. Detached whorl medium to long in length, curved, descending, and twisted. Breathing device knob-shaped. Shell colour uniformly brown to purplish or sometimes with brownish zigzag pattern. Operculum calcareous, and tall cup-shaped.

Riostoma samuiense can be distinguished from R. haughtoni in having a brown to purplish shell, knob-shaped breathing device, and medium detached-whorl length. In comparison, R. haughtoni has a smaller shell, brown to whitish shell colour, a short detached whorl, and notch-shaped breathing device.

Shell. Shell small, cW 15.1–18.9 mm, cH 8.1–10.7 mm, thick, and nearly flattened to sub-discoidal shape; detached-whorl length 3.5–14.5 mm. Apex acute with dark colouration. Whorls 4 to 5, convex, increasing regularly; suture wide and shallow; last whorl rounded and slender. Shell surface with fine growth lines. Periostracum thick, corneous, dark brown, and usually worn down on ventral surface. Shell colour uniformly brown to purplish (sometimes with brownish zigzag pattern); peripheral band usually absent (rarely present). Detached whorl medium to long, ~ 2× apertural width, curved, descending, and sometimes twisted. Peristome circular and double; lip thickened, expanded, and multi-layered. Aperture opened sub-laterally to ventrally. Breathing device knob-shaped; outer lip protruding, with small knob; inner lip with indistinct incision. Umbilicus widely opened and deep. Operculum calcareous, tall cup-shaped, and multispiral (Figs 12–14).

Radula. Teeth arrangement and shape are very similar to R. haughtoni. Central tooth with large central cusp and two lateral cusps on each side. Lateral teeth composed of three to four cusps. Marginal teeth each composed of three cusps (Fig. 16B, C).

The previous records of this species range from the type locality in Samui Island, Gulf of Thailand, to the northern part of peninsular Malaysia, and the southern part of Thailand (Tomlin 1938; Patamakanthin 2001). The northern limit is around Bang Sapan, Prachuapkhirikhan, extending southward along the east coast of the Gulf of Thailand to northern peninsular Malaysia (Fig. 10).

The previous records of both R. samuiense and R. asiphon from “Samui Islands” were inaccurate (see Möllendorff 1893, 1894; Tomlin 1932). This study surveyed entire ranges of the Samui Islands area and found these two species to be allopatric. Rhiostoma asiphon has a distribution restricted to the Angthong Islands (probably the exact type locality), which are the limestone islands ~ 30 km northwest of Samui Island. In contrast, R. samuiense has a narrow range only on the granitic Samui Island (type locality) and its surrounding satellite islands.

Previously, R. chupingense Tomlin, 1938, had long been recognised as a distinct species mainly distributed in northern peninsular Malaysia and southern Thailand (Berry 1963; Sutcharit et al. 2019). The COI barcoding of the topotype of R. chupingense together with other populations from southern Thailand and the topotype of R. samuiense indicate these specimens cluster in a monophyletic clade and with low genetic distance (Figs 3, 4; Appendix 1: Table A1). Therefore, we consider R. chupingense Tomlin, 1938 as a junior subjective synonym of R. samuiense. This new assignment has made the distribution range of R. samuiense much broader on the Malay Peninsula.

Morphological variation occurs in R. samuiense, especially in the length of the detached whorl, which can be divided into three morphotypes:

1. The “typical” morphotype tends to have medium detached-whorl length, approximately the same as apertural width. The specimens tend to have thickened and brownish periostracum. Currently, this typical morphotype occurs only from the type locality.
2. The “ chupingense” morphotype is similar to the “typical” but has a long detached whorl (~ 2× longer than aperture width; detached-whorl length 11.0–18.0 mm, aperture width 6.1–8.1 mm) that is curved, descending and turned so that aperture opens sub-laterally. Some populations tend to have obvious brownish zigzag colour patterns. This morphotype has a wide distribution ranging from Perlis, Malaysia, to the southern part of Thailand and one locality in Myanmar.
3. The “proboscis” morphotype exhibits a markedly long detached whorl (~ 3× longer than aperture width; detached-whorl length 16.0–24.0 mm, aperture width 7.3–8.4 mm) that is curved, descending, and twisted, which turns the aperture to open ventrally. This morphotype is restricted to a few localities in the northern part of the Khao Luang Mountains.

Holotype CUMZ 4709 (cW 21.0 mm, cH 11.8 mm, dL 22.7 mm; Fig. 15A). Paratypes CUMZ 3858 (25 adults + 12 juveniles), CUMZ 4710 (2 shells; Figs 15B, C, 16D), CUMZ 4823 (5 shells), NHMUK 20220438 (5 shells), and SMF 368673 (5 shells). All paratypes are from the type locality.

Figure 15. 

Shell of Rhiostoma rhothonotaphrosa sp. nov. A holotype CUMZ 4709 from Klong Had, Srakeo B, C paratypes CUMZ 4710 from type locality D, E specimens CUMZ 4711 from Khao Chamao, Rayong F specimen CUMZ 4895 from Khao Chakan (north site), Srakeo.

Figure 16. 

Radula morphology A Rhiostoma haughtoni specimen CUMZ 10048/2 from Dhammatat Cave, Mon, Myanmar B, C Rhiostoma samuiense B specimen CUMZ 3989 from Samui Island, Surat Thani and C specimen CUMZ 3996 from Bukit Chuping, Perlis, Malaysia D Rhiostoma rhothonotaphrosa sp. nov. paratype CUMZ 4710 from Klong Had, Srakeo.

Tham Sri Thong, Khlong Had District, Srakeo Province, Thailand (13°28'43.6"N, 102°16'53.8"E). Limestone outcrop with dry deciduous forest, surrounded by sugarcane and tapioca farms.

Thailand: Khao Wong Kot, Kaeng Hang Maeo, Chanthaburi: CUMZ 4852, 4874, 4877, 10174, 10176. Khao Cha Ang Oun, Bo Thong, Chonburi: CUMZ 4363, 4364. Khao Chamao-Khao Wong, Khao Chamao, Rayong: CUMZ 3808, 3857, 3923, 4337, 4368, 4711 (Fig. 15D, E), 10177, 10178. Khao Wong, Khao Chamao, Rayong: CUMZ 4725. Limestone near Khao Chamao Khao Wong, Khao Chamao, Rayong: CUMZ 3851. Khao Chakan, Khao Chakan, Srakeo: CUMZ 3806, 4369, 4370, 4735, 4820, 4829, 4882, 4895 (Fig. 15F), 4898, 10173, 10179. Tham Mahasombat, Khao Chakan, Srakeo: CUMZ 3830. Khao Siwa, Khlong Had, Srakeo: CUMZ 4371. Tham Leam, Khlong Had, Srakeo: CUMZ 3852. Tham Phet Po Thong, Khlong Had, Srakeo: CUMZ 4856, 4878, 10172. Wat Khao Pha Pheung, Khlong Had, Srakeo: CUMZ 10180. Ta Phraya, Srakeo: CUMZ 4862. Tham Khao Phlapphueng Thong, Wang Som Bun, Srakeo: CUMZ 10175.

Shell thick and flattened. Detached whorl long, curved, and descending. Breathing device notch-shaped. Shell colour whitish with brownish zigzag pattern. Operculum calcareous, tall and cup shaped.

This new species differs from R. samuiense from peninsular Thailand by having a notch-shaped breathing device and white shell with brownish zigzag colour pattern. In contrast, R. samuiense has a knob-shaped breathing device and generally monochrome brown to purplish shell. Although this new species has a notch-shaped breathing device similar to R. haughtoni, the distinct characters are longer detached whorl and dark apex, while R. haughtoni has a shorter detached whorl and without dark colour on apex. In addition, R. rhothonotaphrosa sp. nov. differs from R. housei and R. simplicilabre by having a notch-shaped breathing device and relatively longer and descending detached whorl. In contrast, these two species possess tubular breathing devices and a relatively shorter detached whorl.

Shell. Shell medium, cW 19.8–24.9 mm, cH 9.7–12.5 mm, detached-whorl length 10.0–18.5 mm, thick, and sub-discoidal shape. Apex acute with dark colouration; spire slightly convex. Whorls 5 to 6, convex, increasing regularly; suture depressed and deep; last whorl rounded and slender. Shell surface with fine growth lines. Periostracum thick and brown. Shell with brownish zigzag pattern or sometimes with uniformly brownish colour; ventral surface with paler colour; with narrow dark brown spiral band on periphery. Detached whorl long, nearly 2× longer than apertural width, curved and descending. Peristome circular and double; lip thickened, slightly expanded, and multi-layered. Aperture opened sub-ventrally. Breathing device notch-shaped; outer lip protruding, with narrow groove; inner lip with shallow to deep incision. Umbilicus widely opened and deep. Operculum calcareous, tall cup-shaped, and multispiral (Fig. 15).

Radula. Teeth arrangement and shape are almost identical to R. haughtoni. Central tooth with large central cusp and two lateral cusps on each side. Lateral teeth composed of three or four cusps. Marginal teeth each composed of three cusps (Fig. 16D).

The specific name rhothonotaphrosa comes from two Greek words, rhothon meaning nose and taphros meaning ditch or trench. It refers to the notch-shaped breathing device of this new species.

This new species is known from several localities in eastern Thailand near the Cambodia border in Srakeo, Chanthaburi, Rayong, and Chonburi provinces.

Four species, R. hainesi, R. cambodjense, R. rhothonotaphrosa sp. nov., and R. cheliopegma sp. nov., are distributed in eastern Thailand. Our survey throughout their distribution ranges revealed no sympatric populations except for a single locality with adjacent populations of R. cambodjense and R. rhothonotaphrosa sp. nov. These two species inhabit a small limestone outcrop of Khao Chakan, Srakeo Province; the outcrop is ~ 1 km long, 0.5 km wide, and oriented east-west. The new species occupies the northern side of the outcrop, while R. cambodjense commonly occurs on the southern flank of the same mountain. However, these two species are totally different in shell morphology, and their COI barcoding suggests they are not closely related taxa (Fig. 3).

This species was described from specimens from the Haines collection (AMNH). The unique name-bearing type was not designated; however, at least two specimens were illustrated in the original description. The specimen identical to the illustration (shape of breathing device) and measurements in Haines (1855: figs 12, 13) is figured herein, syntype AMNH 42923 (5 shells; Fig. 19A, B) from Siam [Thailand].

Thailand: Boong Tuey, East Siam: RBINS 659990, 659991; Pak Chong, East Siam: RBINS 659978, 659979, 659980; Lam Ton Lang, East Siam: RBINS 659982, 659983, 659984. Siam: NHMW 75000/E/23126 (1 shell). Samui, Siam: NHMW Rusnov (1 shell). Tham Phet Tham Thong, Takhli, Nakhonsawan: CUMZ 4385. Pukae Botanical Garden, Chaloem Phra Kiat, Saraburi: CUMZ 4742. Wat Tham Srivilai, Chaloem Phra Kiat, Saraburi: CUMZ 3982 (Figs 7A–C, 19D, 20, 32A), 4449. Kaeng Khoi, Saraburi: CUMZ 4315. Ban Tha Sao Village, Kaeng Khoi, Saraburi: CUMZ 10135. Wat Tham Phra Phothisat, Kaeng Khoi, Saraburi: CUMZ 10026, 10158. Chet Sao Noi Waterfall, Muaklek, Saraburi: CUMZ 4732. Muaklek, Saraburi: CUMZ 4390, 4460. Wat Tham Dao Khao Kaew, Muaklek, Saraburi: CUMZ 3802, 3934, 4389, 4760, 4842, 4897, 10131, 10133. Khao Sompod, Chai Ba Dan, Lopburi: CUMZ 3932, 3936, 3937. Wat Tham Promlok, Chai Ba Dan, Lopburi: CUMZ 10128. Sub Langka, Lamsonthi, Lopburi: CUMZ 4736. Khao Look Chang, Pak Chong, Nakhon Ratchasima: CUMZ 3805, 3981, 4316, 4318, 4320, 4379, 4384 (Fig. 19E), 4454. The way to Khao Yai, Pak Chong, Nakhon Ratchasima: CUMZ 4454. Wat Tham Prommachan Thammaram, Pak Chong, Nakhon Ratchasima: CUMZ 10160. Wat Tham Sub Muet, Pak Chong, Nakhon Ratchasima: CUMZ 10132. Wat Tham Thep Nimit, Pak Chong, Nakhon Ratchasima: CUMZ 10126. Wat Thepphitak Punnaram, Pak Chong, Nakhon Ratchasima: CUMZ 4321, 4416. Sakaerat Biosphere, Pak Thong Chai, Nakhon Ratchasima: CUMZ 4391. Sakaerat Environmental Research Station, Wang Nam Khiao, Nakhon Ratchasima: CUMZ 10134. Ban Bueng, Ban Bueng, Chonburi: CUMZ 4312. Khao Chakan, Srakeo: CUMZ 10156. Khao Nang Panturat, Cha Am, Phetchaburi: CUMZ 4879 (Fig. 19F), 10129, 10130, 10164, 10165. Khao Nang Panturat Nature Reserve, Cha Am, Phetchaburi: CUMZ 4480. Tham Na Kwang, Cha Am, Phetchaburi: CUMZ 3988, 4452, 4719, 10162. 30 km. before Pa La Au Waterfall, Hua Hin, Phetchaburi: CUMZ 3827, 4740. Kaeng Kachan, Phetchaburi: CUMZ 4409, 4412. Khao Lom Muak, Muang, Prachuap Khiri Khan: CUMZ 3870, 3987 (Fig. 17A), 4322, 10163. Bang Poo, Sam Roi Yod, Prachuap Khiri Khan: CUMZ 4414, 4417, 4429, 4430. Saun Wiweg Bureau of Monks, Sam Roi Yod, Prachuap Khiri Khan: CUMZ 10127. Ao Noi, Prachuap Khiri Khan: CUMZ 3868. Tham Pissadarn, Tha Sae, Chumphon: CUMZ 4383 (Fig. 19C), 4453, 4851.

Figure 17. 

Live snails of the species group II A Rhiostoma housei specimen CUMZ 3987 from Ao Manow, Prachuap Khiri Khan B, C Rhiostoma hainesi from Khao Soi Dao, Chanthaburi B without breathing device and C with long breathing device D Rhiostoma jalorensis specimen CUMZ 3994 from Wat Tham Sue, Krabi. All figures are not to scale. Photographs: P. Tongkerd (B, C).

Shell medium to large, thick, and depressed. Detached whorl longer than aperture width, curved, and descending. Breathing device straight with complete tube. Shell usually with dark brown zigzag patterns.

Rhiostoma housei can be distinguished from R. tigrina sp. nov. in usually having a stout detached whorl, generally a curved tubular breathing device, and a faded colour pattern. In contrast, R. tigrina sp. nov. has a slender detached whorl, generally a straight tubular breathing device, and a brownish zigzag colour pattern.

Shell. Shell medium to large, cW 22.1–26.4 mm, cH 12.1–13.8 mm, thick, and sub-discoidal to discoidal shape; detached-whorl length 8.0–14.5 mm. Apex acute with dark colouration; spire slightly elevated. Whorls 5 to 6, convex and increasing regularly; suture wide and deep; last whorl rounded and stout. Shell surface with fine growth lines. Periostracum thin corneous, and transparent to thick with brownish colour. Shell colour whitish with dark brown zigzag pattern; dorsal shell surface with darker colour pattern than ventral surface; with narrow brownish spiral band on periphery (sometimes absent). Detached whorl of medium length, approximately the same as apertural width. Peristome circular and double; lip thickened, slightly expanded and multi-layered. Aperture opened sub-laterally. Breathing device tubular and its tip usually attached to preceding whorl; outer lip forms a long, curved or straight, closed tube; inner lip with small hole inside aperture. Umbilicus widely opened and deep. Operculum calcareous, tall cup-shaped, and multispiral (Fig. 19).

Radula. Taenioglossate radula arranged in inverted V-shaped rows. Central tooth with well-developed central cusp and two lateral cusps on each side tapering in size; central cusp large with blunt tip; lateral cusps with triangular shape and pointed tip. Lateral teeth consisting of four cusps; central cusp large, elongate, and flanked by pointed tips of two small inner cusps and one outer cusp. Inner and outer marginal teeth each composed of three cusps; central cusp with large and dull tip, flanked by small pointed tips of one inner cusp and one outer cusp (Fig. 32A).

The type locality was given as a broad location of “Siam”; however, the specimens from central Thailand are the most similar to the syntype illustrated herein (Fig. 19A, B). In this study, R. housei has a wide distribution ranging from central to peninsular Thailand. The southernmost limits are around the Isthmus of Kra (Pratiew, Chumphon Province), and the northern limit is approximately in Nakhon Sawan Province (Fig. 18).

Figure 18. 

Distribution map of Rhiostoma housei–species group.

Figure 19. 

Shell of Rhiostoma housei A, B syntype AMNH 42923 from Siam A figured specimen in Haines (1855: figs 12, 13) anb B other specimen from the same syntype lot C specimens CUMZ 4383 from Tham Phitsadan, Chumphon D specimen CUMZ 3982 from Tham Sriwilai, Saraburi E specimen CUMZ 4384 from Pak Chong, Nakhon Ratchasrima F specimen CUMZ 4879 from Hua Hin, Prachuap Khiri Khan.

Figure 20. 

Drawing of general anatomy of Rhiostoma housei specimen CUMZ 3982 from Saraburi Province, Thailand. A female B male. Abbreviations: bc, bursa copulatrix; ; cm, columellar muscle; ct, cephalic tentacle; dg, digestive gland; es, eyespot; f, foot; fc, faeces; int, intestine; lc, lung cavity; mc, mantle collar; od, oviduct; op, operculum; ov, ovary; p, external penis; pg, prostate gland; sg, sperm groove; sn, snout; te, testis; ut, uterus; vd, vas deferens; vg, vaginal groove.

Previous literature recorded R. housei from several places in Laos, Cambodia, and Vietnam [Tonkin] (Möllendorff 1885; Ancey 1898; Fischer 1973). However, these records seem ambiguous because the type specimens have never been examined, were very limited in the number of specimens, and many species have a similar shell morphology to R. housei. Moreover, the species delimitation by previous authors is probably different from the concept of the type specimens. For example, this species has been reported in Chiangdao, Chiangmai, northern Thailand (sensu Solem 1966). We collected many specimens from several populations in the same area and examined Solem’s specimens in ZMUC. Due to the clear differences in shell characters, we recognise these populations as distinct species (R. tigrina sp. nov.). For that reason, the distribution range of R. housei, especially from Laos, Cambodia, and Vietnam, needs to be re-examined with new specimen collections with more precise locality.

Do et al. (2020b) reported several “R. housei” populations from Northern Vietnam in Dien Bien, Hao Binh, and Son La provinces. The figured specimen from Dien Bien Province shows the distance ~800 km away from the central population of this species from central Thailand. This specimen (Do et al. 2020b: fig. 6b, c) had a depressed spire, and a curved, tubular breathing device, and is probably R. simplicilabre rather than R. housei.

The specimens collected from southern Thailand (Chumporn, Phetchaburi, and Prachuapkhirikhan provinces) tended to have longer detached whorls, longer breathing devices, and brighter shell colour than the populations from central Thailand (typical). However, the shell colour pattern, detached whorl, and breathing device structures are identical to the type specimens (Fig. 19A, B). Therefore, these are attributed to morphological variation across the wide distribution range of this species.

Syntype NHMUK 20170371 (2 shells; Fig. 21A) from Camboja. Holotype NMNH 382943 (Fig. 21B) of Rhiostoma smithi Bartsch, 1932, from Kao Sabap, S.E. Siam. Holotype NMW 1955.158.24924 (Fig. 21C) of Rhiostoma tomlini Salisbury, 1949, from Khao Sabap, Thailand, paratype NHMUK 1949.6.7.1 (2 shells; Fig. 21D) from Khao Sabap, Siam, and paratype NHMUK 20170372 (1 juvenile shell) from Khao Sabap, Siam. Holotype NSMT 52242 (Fig. 21E) of Rhiostoma housei kirai Habe, 1965, from Chanthaburi, Thailand.

Thailand: Khao Yai, Muang, Nakornnayok: CUMZ 4458. Khoa Sib Ha Chun, Kaeng Hang Maeo, Chanthaburi: CUMZ 10075, 10107. Klong Plub, Khlung, Chanthaburi: CUMZ 10017. Makok Waterfall, Khlung, Chanthaburi: CUMZ 3807, 3814, 3920, 3921, 4421, 4814 (Fig. 22B), 4819, 4827, 10102, 10105, 10109. Trok Nong Waterfall, Khlung, Chanthaburi: CUMZ 4455. Ban Ma Kham, Ma Kham, Chanthaburi: CUMZ 10099. Phrabat Khao Ma Kham, Ma Kham, Chanthaburi: CUMZ 10116. Khlong Narai waterfall, Muang, Chanthaburi: CUMZ 10101. Plieu Waterfall, Chanthaburi: CUMZ 3801, 4336, 4338, 4339, 4340, 4380, 4381 (Figs 21F, 22A), 4393, 4394, 4403, 4404, 4405, 4418, 4422, 4425, 4426, 4428, 4434, 10106, 10194 (Fig. 22D). Si Bha Chan Waterfall, Muang, Chanthaburi: CUMZ 4733. Hin Dat waterfall, Pong Nam Ron, Chanthaburi: CUMZ 10100. Khao Soi Dao Wildlife Breeding Center, Soi Dao, Chanthaburi: CUMZ 3855, 3883, 3898, 3917, 4402, 4423, 4457, 4743, 10114, 10117. Wat Khao Sukim, Tha Mai, Chanthaburi: CUMZ 3803, 3815, 3828, 3918, 3919, 4867, 10103, 10108, 10110, 10111, 10118, 10119, 10120. Wat Tham Klong Tip, Tha Mai, Chanthaburi: CUMZ 10112. Chanthaburi: CUMZ 10113. Khao Chamao-Khao Wong, Khao Chamao, Rayong: CUMZ 4456, 4816 (Fig. 22C), 4822, 10098. Khao Chamao Waterfall, Khao Chamao, Rayong: CUMZ 3897. Klong Pla Kang waterfall, Khao Chamao-Khao Wong, Khao Chamao, Rayong: CUMZ 10104. Wat Chat San Phatthanaram, Bo Rai, Trat: CUMZ 10115.

Figure 21. 

Shell of Rhiostoma hainesi A syntype NHMUK 20170371 from Camboja B holotype of “Rhiosotma smithi Bartsch, 1932” NMNH 382943 from Khao Sabap, southern Siam C, D originally labelled as “Rhiostoma tomlini Salisbury, 1949” from Khao Sabap, Siam C holotype NMW 1955.158.24924 and D paratype NHMUK 1949.6.7.1 E holotype of “Rhiostoma housei kirai Habe, 1965” NSMT 52242 from Chanthaburi, Thailand F specimen CUMZ 4381 from Chanthaburi.

Figure 22. 

Shell of Rhiostoma hainesi A specimen CUMZ 4381 from Chanthaburi B specimen CUMZ 4814 from Makok Waterfall, Chanthaburi C specimen CUMZ 4816 from Khao Chamao, Rayong D specimens CUMZ 10194 from Chanthaburi shows series of growth from small juvenile (on the left) to nearly adult (on the right).

Shell large, thick, and depressed. Detached whorl longer than aperture width, curved and descending. Apertural lip much expanded on palatal wall. Breathing device usually tubular and its tip often attached to preceding whorl. Shell usually with thick dark brown periostracum.

Rhiostoma hainesi differs from R. rhothonotaphrosa sp. nov. by having a large and stout last whorl, tubular- or notch-shaped breathing device, thickened brownish periostracum, and lip slightly thickened and widely expanded. In contrast, R. rhothonotaphrosa sp. nov. has a smaller shell size, slender last whorl, notch-shaped breathing device, white shell colour with brownish zigzag pattern, periostracum thin corneous, and lip thickened, slightly expanded, and multi-layered.

Shell. Shell large, cW 22.1–27.8 mm, cH 11.3–15.2 mm, thick, and discoidal shape; detached-whorl length 11.0–28.5 mm. Apex acute; spire elevated. Whorls 5 to 6, convex, increasing regularly; suture wide and slightly deep; last whorl rounded and stout. Shell surface with fine growth lines. Periostracum thick corneous with brownish to dark brown colour (without periostracum shell uniformly whitish to yellowish colour); with narrow dark spiral band on periphery. Detached whorl long, ~2× longer than apertural width, curved and descending. Peristome circular and double; lip thickened, broadly expanded on palatal wall, and multi-layered. Aperture opened sub-laterally to sub-ventrally. Breathing device typically tubular (incomplete tube and notch shape sometime present), and its tip sometimes attached to preceding whorl; outer lip forms a short to long closed tube and is located just behind apertural lip; inner lip with small hole inside aperture (or deep incision in notch shape). Umbilicus widely opened and deep. Operculum calcareous, cup-shaped, and multispiral (Figs 21, 22).

Radula. Teeth arrangement and shape are almost identical to R. housei. Central tooth with large central cusp and two lateral cusps on each side. Lateral teeth composed of four cusps; central cusp large with blunt tip; two inner cusps with pointed tips; one outer cusp small and dull. Marginal teeth each consisting of three pointed cusps (Fig. 32B).

The previous records of R. hainesi were mostly from vague locations in Cambodia and Thailand: Khao Sabap (= Plieu National Park, Chanthaburi) (Pfeiffer 1862a; Habe 1965). The recent study obtains specimens from several localities in Nakhonnayok, Rayong, and Chanthaburi provinces (Fig. 18).

Three nominal taxa have long been confused with R. hainesi: R. smithi Bartsch, 1932, R. tomlini Salisbury, 1949, and R. housei kirai Habe, 1965. The first two species were described based only on differences in shell colour and shapes of the breathing device. The type localities of both are from the same location at “Khao Sabap” [= Plieu National Park, Khlung, Chanthaburi, Thailand], and their holotypes are identical with R. hainesi (Fig. 21A–D). The last taxon, R. housei kirai, was nominated as a subspecies of R. housei (sensu Habe 1965). However, the holotype (Fig. 21E) exhibits a long and curved detached whorl, short tubular breathing device, expanded and thick apertural lip, and brownish periostracum (remaining inside umbilical area), which are the unique characters of R. hainesi. In addition, the type locality was Chanthaburi Province, which is within the distribution range of R. hainesi. Therefore, we recognise these three nominal taxa as junior subjective synonyms of R. hainesi.

The three variations in breathing device shape were reported in Tumpeesuwan (2001: fig. 5.2d–f): tubeless, incomplete tube, and complete tube. In this study, R. hainesi from Chanthaburi populations also exhibit sympatrically occurring morphs. The typical and more abundant morph has a short to long tubular breathing device (Figs 21F, 22A), and the less abundant morph has a notch-shaped breathing device (Fig. 22C). Specimens with these three distinct types of breathing devices were sampled for the COI analysis, and the results suggest that these are conspecific (Fig. 3).

Syntype NHMUK 20130214 (4 shells; Fig. 23A) from Camboja [Cambodia]

Laos: Tham Pha Tok, Nong Khiaw, Luang Phrabang: CUMZ 10010/1, 10010/2 (Fig. 23E). Ban Oudom, Meuang Xai, Oudomxay: CUMZ 10006 (Fig. 23D). Ban Homsai, Meuang Ngeun, Sayaboury: CUMZ 10007, 10008. Ban Na Mone, Sayaboury: CUMZ 10009/1, 10009/2 (Fig. 23B, C). Thailand: Sa-pan Waterfall, Bo Kluea, Nan: CUMZ 4868 (Fig. 23F).

Figure 23. 

Shell of Rhiostoma simplicilabre A syntype NHMUK 20130214 from Camboja B, C specimens CUMZ 10009/2 from Ngeun, Sayaboury, Laos D specimen CUMZ 10006 from Meuang Xai, Oudomxay, Laos E specimen CUMZ 10010/2 from Nong Khiaw, Luang Prabang, Laos F specimen CUMZ 4868 Bor Kluea, Nan.

Shell medium, thin, and depressed. Detached whorl longer than aperture width and descending. Breathing device tubular, and often attached to preceding whorl. Shell with brownish zigzag patterns; thick corneous periostracum.

Based on the type and museum specimens, R. simplicilabre is very closely allied to R. housei. The differences are the tubular-shaped breathing device perpendicular to the detached whorl, depressed spire, and thickened periostracum. It differs from R. hainesi by having a brownish zigzag colour pattern, shorter detached whorl, and smaller shell size. In comparison, R. hainesi has a thick brownish to dark brown periostracum, longer detached whorl, and larger shell size.

Shell. Shell medium, cW 20.9–26.1 mm, cH 9.7–12.8 mm, thin, and sub-discoidal to discoidal shape; detached-whorl length 7.5–12.5 mm. Apex acute; spire slightly elevated. Whorls 5 to 6, convex, increasing regularly; suture wide and deep; last whorl rounded and stout. Shell surface with fine and regular growth lines. Periostracum thick corneous, and transparent. Shell with brownish zigzag patterns; ventral shell surface with paler colour and less brownish pattern; with narrow dark brown spiral band on periphery. Detached whorl long and almost same length as apertural width, curved and descending. Peristome circular and double; lip somewhat thin and slightly expanded. Aperture opened sub-laterally. Breathing device tubular and its tip usually attached to preceding whorl; outer lip forming short, curved, closed tube located just behind apertural lip; inner lip with small hole inside aperture. Umbilicus widely opened and deep. Operculum calcareous, cup-shaped, and multispiral (Fig. 23).

Figure 24. 

A specimen mentioned as being the syntype of “Pterocyclos marioni Ancey, 1898” NMW 1955.158.24090. B–F shell of Rhiostoma marioni B published figure of the type specimen (after Ancey 1898: pl. 9, fig. f.) and C–F specimens CUMZ 10210 from Feuang, Vientiane, Laos.

None of the R. simplicilabre specimens were collected from modern-day Cambodia; therefore, the distribution ranges of this species still need to be verified with specimens from accurate localities within Cambodia (Fig. 18).

In this study, R. simplicilabre has been determined to be mainly distributed in the northern part of Laos in Luang Phrabang and Xayabuli provinces, and in northern Thailand. In addition, the specimens recorded from Son La, Dien Bien, and Hao Binh provinces in northern Vietnam have been attributed to R. housei.

This species was described in the same publication by L. Pfeiffer (1862a), immediately after R. hainesi based on specimens in the collection of H. Cuming obtained from Henry Mouhot. The collection locality was brief “Camboja”; it is known that Mouhot’s recorded localities were imprecise and referred to broad geographical areas, for example, “Siam”, “Lao Mountains, Camboja” and “Camboja” (see Páll-Gergely et al. 2016a; Inkhavilay et al. 2019; Pholyotha et al. 2021). This has made it challenging to infer more precise type localities of several land snail species described from Mouhot’s specimens. According to the recorded itinerary of H. Mouhot (Mouhot 1864: 57; Ashburton 1864: map), land snail surveys from southern Cambodia could not find any specimens matching this species. The most recent works on land snails from southern Cambodia suggest the non-existence of R. simplicilabre (see Sutcharit et al. 2020). We have surveyed several limestone hills near Luang Phrabang areas, Laos, and encountered numbers of empty shells and living specimens that are well-matched with the type specimens of R. simplicilabre in all aspects (Fig. 23B–E). In addition, the specimens referred to as “Rhiostoma marioni (Ancey, 1898)” in Inkhavilay et al. (2019: fig. 11b) should be recognised as this species.

The figured specimens in Ancey (1898: pl. 9, fig. f) could not be located.

Laos: Wat Non Hinhare, Feuang District, Vientiane Province: CUMZ 10210 (7 shells; Fig. 24C–F)

Shell medium, thick and depressed shell. Detached whorl long, ~2× longer than aperture width, curved, slightly descending. Breathing device tubular and straight. Shell colour uniformly brownish or with irregular blotches.

Rhiostoma marioni has been considered a junior synonym of R. housei from Thailand (Kobelt 1911). However, this species differs from R. housei in having a monochrome brownish shell colour, long detached whorl, and long straight tubular breathing device. In comparison, R. housei tends to have a brownish zigzag pattern, medium detached-whorl length, and a curved breathing tube.

Shell. Shell medium, cW 17.1–20.5 mm, cH 9.1–10.6 mm, thick, and sub-discoidal to discoidal shape; detached-whorl length 14.0–20.5 mm. Apex acute; spire slightly elevated. Whorls 5 to 6, convex, increasing regularly; suture wide and deep; last whorl rounded and stout. Shell surface with fine growth lines. Periostracum thin corneous, and transparent. Shell colour uniformly brownish to dark brown (rarely with brownish blotches); ventral shell surface with paler colour; with narrow and dark spiral band on periphery. Detached whorl long, ~ 2× longer than apertural width, curved and descending. Peristome circular and double; lip thickened, expanded and multi-layered. Aperture opened sub-ventrally. Breathing device tubular and its tip often attached to preceding whorl; outer lip forming a long, straight, closed tube, located just behind apertural lip; inner lip with small hole inside aperture. Umbilicus widely opened and deep. Operculum calcareous, cup-shaped, and multispiral (Fig. 24B–F).

This species is currently known only from Vientiane Province, Laos (Fig. 18).

This species was described based on two specimen lots from “Luang-prabang, Laos” and “Mont Hou, Tonkin” [probably in the area of Muang Khua and Yot Ou Districts, Phongsaly Province, Laos (see Pavie 1903: pl. 1)]. The original description includes an illustration of a single specimen and one set of measurements (Ancey 1898). Unfortunately, this figured specimen could not be located from the RBINS or NMW collection. The specimen lot NMW 1955.158.24090 consisting of a single specimen (with operculum) that has Ancey’s type label and collection locality “Luang-prabang” was considered the possible holotype by Wood and Gallichan (2008: pl. 25, figs 3, iv). However, this surviving specimen looks different from the actual illustrated specimen in the original publication. This specimen has a flattened shell, short detached whorl, and notch-shaped breathing device projecting anteriorly (Fig. 24A). Although the operculum is not clearly mentioned in the original description, the operculum accompanied by this specimen lot (NMW 1955.158.24090) is dark corneous, plate-shaped and multispiral with elevated lamellae (Fig. 24A). It is distinct from the typical operculum of Rhiostoma (Fig. 2), and more closely resembling the Ptychopoma or Scabrina. This specimen is contradicting to Ancey’s concept of “Pterocyclos marioni”, whereas the species description agrees well with the specimen illustration (Ancey 1898: pl. 9, fig. f). Therefore, we consider specimen NMW 1955.158.24090 (Fig. 24A; Inkhavilay et al. 2019: fig. 11a; Do et al. 2020b: fig. 1d) as a distinct species. We have excluded that specimen from our attention in the type series.

This redescription is based on the recently collected specimens from Feuang District, Vientiane Province, Laos. Before establishing the French Protectorate of Laos in 1893 (Pavie 1903: pl. 6; Tips 1999: 131), the historical name “Luang Prabang” generally referred to the city or the Kingdom Luang Prabang (during the 18^th to 19^th century). This name encompasses a wide geographical area, mainly present-day northern Laos: Bokeo, Houa Phanh, Luang Namtha, Luang Prabang, Oudomxay, Phongsaly and Vientiane provinces. Therefore, this newly collected specimen from Vientiane Province is considered the topotypic specimen of this species. This specimen lot possesses a long detached whorl and a long tubular-shaped breathing device identical to the unique characteristic of R. marioni; it agrees well with the illustration (Fig. 24B) and the species description.

The previous records under the name Rhiostoma “marioni” from Lai Chu, Son La, Dien Bien and Lau Cai provinces, North Vietnam, as mentioned by Do et al. (2020b), should be referred to another species (see under R. abletti).

Syntype NHMUK 1902.1.24.14–16 (3 shells; Fig. 25A) from Pitsunaloke, Siam. This syntype lot in the Blanford ex. Daly collection consists of three specimens; the figured specimen in the original description can be recognised by its curved breathing device and is figured herein.

Figure 25. 

Shell of Rhiostoma dalyi A syntype NHMUK 1902.1.24.14–16 from Juxta Phitsanulok B, C specimens CUMZ 4848 from Tham Erawan, Long, Prae D specimen CUMZ 3979 from Tarnsawan Waterfall, Phayao E, F specimens CUMZ 3926 from Tham Pha Tangyai, Pong, Phayao.

Thailand: Tham Pha Dang, Chiangkham, Phayao: CUMZ 4462, 4729. Doi Phunang, Dokkamtai, Phayao: CUMZ 3938, 3942, 4313, 4319, 4420. Tarnsawan Waterfall, Dokkamtai, Phayao: CUMZ 3824, 3826, 3977 (Fig. 25D). Tham Pha Tangyai, Pong, Phayao: CUMZ 3926 (Fig. 25E, F), 4784. Tham Pha Tung, Pong, Phayao: CUMZ 10062. Ban Tha Sri, Muang, Lampang: CUMZ 4728. Pratupha, Ngoaw, Lampang: CUMZ 4463. 3 km. before the junction to Tham Sereethai, Long, Prae: CUMZ 4450. Hill Near Tham Air Tammachard, Long, Prae: CUMZ 3979, 4451. Tham Air Thammachad, Ban Pan, Long, Prae: CUMZ 4762. Tham Erawan, Long, Prae: CUMZ 4792, 4848 (Fig. 25B, C), 10063, 10064. Wat Tham Pha Nang Koy, Rong Kwang, Prae: CUMZ 3884, 3924, 4726.

Shell large, thick, and depressed. Detached whorl medium in length, and slightly descending. Breathing device tubular, curved, and attached to preceding whorl. Shell whitish with brownish zigzag patterns.

This species is very similar to R. housei from central Thailand. The distinguishing characteristics of R. dalyi are a stout last whorl and a tubular-shaped breathing device that is short, curved and attached to the preceding whorl. In comparison, R. housei has a slender last whorl, and a long and straight tubular breathing device. In addition, this species also differs from R. trigina sp. nov. in having a stout and short detached last whorl, a short and curved tubular breathing device, and a ventral shell surface usually without a pattern. In contrast, R. trigina sp. nov. possesses a slender detached whorl, long and straight tubular breathing device, and usually with brown zigzag patterns on both sides.

Shell. Shell medium to large, cW 19.9–24.6 mm, cH 9.7–13.4 mm, thick, and discoidal shape; detached-whorl length 7.5–14.0 mm. Apex acute with dark colouration; spire slightly elevated. Whorls 5 to 6, convex and increasing regularly; suture wide and deep; last whorl rounded and stout. Shell surface with fine growth lines. Periostracum thick or thin corneous and transparent. Shell colour whitish with brownish zigzag pattern; ventral shell surface usually without pattern; with narrow and dark brown spiral band on periphery. Detached whorl of medium length, approximately the same length as apertural width. Peristome circular and double; lip thickened, slightly expanded and multi-layered. Aperture opened sub-laterally. Breathing device tubular, curved and its tip attached to preceding whorl; outer lip forming a short and closed tube, and located just behind apertural lip; inner lip with hole inside aperture. Umbilicus widely opened and deep. Operculum calcareous, cup-shaped, and multispiral (Fig. 25).

This species is distributed in the northern part of Thailand in Phitsanuloke (type locality), Lampang, Phrae and Phayao provinces (Fig. 18).

Solem (1966) record the specimens from ‘Chieng Dao’ as this species. However, we examined a number of specimens from Chieng Dao, Chieng Mai Province and found clearly distinct shell morphology compared to the type specimen of R. dalyi. In addition, the COI barcoding suggests that R. dalyi and R. trigina sp. nov. are separate species (Fig. 3). Therefore, the record of this species by Solem (1966) should be referred to R. trigina sp. nov.

Holotype UMZC I.100155 (Fig. 26A) from Biserat, State of Jalor, Malay Peninsula and paratype NMW 2.1981.118.02760 (1 shell; Fig. 26B). Holotype MNHN-IM-2000-34049 (Fig. 26C) of Rhiostoma huberi Thach, 2018, from Ao Luc, South Thailand.

Figure 26. 

Shell of Rhiostoma jalorensis A holotype UMCZ 1030 from Biserat, Jalor B paratype NMW 2.1981.118.02760 from type locality C holotype of “Rhiostoma huberi Thach, 2018” MNHN-IM-2000-34049 from Ao Luc, South Thailand D, E specimens CUMZ 3994 from Wat Tham Sue, Krabi F specimen CUMZ 4387 from Tham Nam Pud, Phang Nga. Photograph: M Caballer and V Heros, MNHN (C).

Thailand: Klong Seang Wildlife Sanctuary, Ban Ta Khun, Surat Thani: CUMZ 4323. Nature Trail, Khaosok, Ban Ta Khun, Surat Thani: CUMZ 4478, 4723. Ratchaprapha Dam, Ban Ta Khun, Surat Thani: CUMZ 4894. Anurak Resort, Panom, Surat Thani: CUMZ 10142. Tham Nam Lod, Panom, Surat Thani: CUMZ 10145. Ao Phang-nga, Muang, Phang Nga: CUMZ 4327. Khao Prao, Muang, Phang Nga: CUMZ 4327. Phang-nga Bay, Muang, Phang Nga: CUMZ 4325, 4326. Wat Tham Suwannakhuha, Takua Thung, Phang Nga: CUMZ 3837, 3848, 4328, 4329, 4821, 4843 (Fig. 27A–C). Tham Nam Pud, Muang, Phang Nga: CUMZ 3836, 3839, 3841, 3899, 4387 (Fig. 26F), 10136. Tham Kiriwong (Tham Kob), Thap Put, Phang Nga: CUMZ 3822, 3823, 3832, 4484, 4485, 4850, 10143. Tao Thong Waterfall, Thap Put, Phang Nga: CUMZ 3817, 3829, 3845, 4386, 4482, 4483 (Fig. 27D–F), 4798, 4837, 4858, 4873, 10141. On the way to Than Bok Khorani, Ao Luek, Krabi: CUMZ 10144, 10146. Tham Na Mee, Ao Luek, Krabi: CUMZ 10137. Tham Phet, Ao Luek, Krabi: CUMZ 3831. Tham Sra Yoon Thong, Ao Luek, Krabi: CUMZ 3819. Tham Chang Sri, Muang, Krabi: CUMZ 4330. Tham Lang Rong Rian, Ban Tubprik Primary School, Muang, Krabi: CUMZ 4487. Wat Tham Sua, Muang, Krabi: CUMZ 3834, 3994 (Figs 17D, 26D, E), 4331, 4486, 4859, 10138, 10139, 10140.

Figure 27. 

Shell of Rhiostoma jalorensis A–C specimens CUMZ 4843 from Wat Tham Suwannakhuha, Phang Nga shows variation of the shell size and length of detached whorl D–F specimens CUMZ 4483 from Tao Thong Waterfall, Phang Nga shows variation in length of the breathing devices.

Shell medium, thick and depressed. Detached whorl long, curved and descending. Breathing device tubular. Shell colour usually uniformly brown to dark brown and with or without peripheral band.

Rhiostoma jalorensis can be distinguished from R. ebenozostera sp. nov. in having longer detached whorl, uniformly dark brown shell colour, and narrow peripheral band. In contrast, the new species has a shorter detached whorl, dark brown blotches shell colour, and with broad and dark brown peripheral band. In addition, the geographically disjunct distributions suggest that they are distinct species.

Shell. Shell small to medium, cW 17.1–20.6 mm, cH8.8–11.8 mm, slightly thick, and sub-discoidal shape; detached-whorl length 12.5–19.5 mm. Apex acute; spire slightly elevated. Whorls 5 to 6, convex, increasing regularly; suture wide and deep; last whorl rounded. Shell surface with fine growth lines. Shell colour uniformly brown, dark brown to black (rarely with zigzag pattern); ventral shell surface with paler colour; thin and narrow peripheral band sometimes present. Periostracum thick or thin corneous and transparent. Aperture opened sub-ventrally to ventrally. Detached whorl long, ~ 2× longer than apertural width, curved and descending. Peristome circular and double; lip thickened, expanded, and multi-layered. Breathing device tubular, straight and its tip usually attached to preceding whorl (sometimes not attached); outer lip forming a long, straight and closed tube, and located just behind apertural lip; inner lip with small hole inside aperture. Umbilicus widely opened and deep. Operculum calcareous, cup-shaped, and multispiral (Figs 26, 27).

Radula. Teeth arrangement and shape are similar to R. housei. Central tooth with large central cusp and two lateral cusps on each side. Lateral teeth composed of four cusps; central cusp large and triangular; two inner cusps with pointed tips and tapering in size; one outer cusp small. Marginal teeth each composed of three cusps (Fig. 32C).

The previous records of this species were from the type locality, Biserat, Jalor (former name of Yala Province, Thailand). The records from Koh Si-Hah, Singgora (now in Phatthalung Province, Thailand) by Laidlaw (1928) and Salisbury (1949) need to be verified with new collections. The recent collections are mainly from the western part of peninsular Thailand in Surat Thani, Phang Nga and Krabi provinces (Fig. 18).

Rhiostoma jalorensis tends to have high variation in both shell form and shell colour. For example, the specimens from Tham Nam Pud, Phang Nga (Fig. 26F) exhibit relatively large shells with long and descending detached whorl (dL = 12.0–25.0 mm, 19.28±3.43 mm), dark shell colour, and aperture opened sub-ventrally to ventrally. On the other hand, the population from Wat Suwankuha, Phang Nga (Fig. 27A–C) tend to have small shells with short detached whorl (dL = 9.0–14.5 mm, 11.67 ± 1.31 mm), pale brown to dark shell colour, and aperture opened sub-laterally. The specimens from Tao Tong Waterfall, Phang Nga (Fig. 27D–F) have medium to long descending detached whorl, brownish shell colour, and aperture opened sub-ventrally. However, the unique attributes of descending detached whorl, tubular-shaped breathing device and monochrome brown to black shell colour suggest that these are morphological variations.

The holotype of Rhiostoma huberi Thach, 2018 from Krabi Province is in an early stage of maturation, as the shell has a thin and sharp apertural lip. This specimen has a long and descending detached whorl, a brownish zigzag pattern on the coiled whorl, tubular-shaped breathing device, and aperture opened ventrally. In the absence of continuous characters in other populations, this extreme example would undoubtedly be recognised as a distinct species. However, the COI analysis clearly suggests that specimens from the entire distribution range and with these shell variations belong to a single species (Fig. 3). Therefore, we formally synonymise Rhiostoma huberi Thach, 2018 with R. jalorensis.

Holotype MNHN-IM-2000-34050 (Fig. 28A) from Thakhek, Central Laos.

Figure 28. 

Shell of Rhiostoma thachi A holotype MNHN-IM-2000-34050 from Thakhek, Laos B–E specimen CUMZ 3941 from Vang Vieng, Vientiane, Laos (E with malform). Photograph: M Caballer and V Heros, MNHN (A).

Laos: Tam Jung (cave), Vang Vieng District, Vientiane Province: CUMZ 3941 (8 shells; Fig. 28B–E), CUMZ 10201 (5 shells).

Shell medium, thick and depressed shell. Detached whorl long, curved and descending, and sutural ridge absent. Breathing device very long, tubular. Shell colour uniformly brown to dark brown along dorsal and ventral sides. Peripheral band absent. Apertural lip much expanded at palatal side, double, with many concentric lines.

Rhiostoma thachi can be distinguished from R. jalorensis from peninsular Thailand in having a very long detached whorl, without peripheral band, and lip much expanded at palatal side. In comparison, R. jalorensis has a shorter detached whorl, with peripheral band, and lip evenly thickened and expanded.

Shell. Shell medium, cW 17.2–21.4 mm, cH 8.8–11.9 mm, thick, and nearly flattened to sub-discoidal shape; detached-whorl length 19.5–27.0 mm. Apex acute; spire slightly elevated. Whorls 5 to 6, convex, increasing regularly; suture wide and deep; last whorl rounded and stout. Shell surface with fine growth lines. Periostracum thin corneous, and transparent. Shell colour uniformly brown to dark brown; ventral shell surface with paler colour; peripheral band absent. Detached whorl long, ~ 2× longer than apertural width, curved and descending, and sutural ridge inconspicuous. Peristome circular and double; lip brownish, thickened, much expanded on palatal side, and multi-layered. Aperture opened sub-laterally to ventrally. Breathing device tubular, and may be attached to preceding whorl; outer lip forming a long and complete tube, located just behind apertural lip; inner lip with hole inside aperture. Umbilicus widely opened and deep. Operculum calcareous, cup-shaped, and multispiral (Fig. 28).

This species is known from the type locality, Khammouane Province, and a limestone mountain in Vientiane Province (Fig. 18).

The empty shells from Vang Vieng District, Vientiane Province, Laos, are relatively bleached and have no operculum. Instead, they have a long, curved and descending detached whorl; tubular-shaped breathing device; aperture opened ventrally; apertural lip brownish, thickened and expanded. However, these unique shell characters suggest they are conspecific with R. thachi from Central Laos. No living specimens of this species were found during this survey. Therefore, more sampling effort and molecular phylogenetic analyses are required to demonstrate the relationship between this species and other species in the R. housei group.

The specimen mentioned as R. haughtoni in Thach (2018: 17, figs 66, 67) is absolutely different from the type specimens from Myanmar (Fig. 11A, B). In addition, this specimen agrees well with all shell characters and was also collected from an area near the R. thachi type locality. Therefore, this specimen is misidentified and should be recognised as R. thachi.

Holotype CUMZ 4703 (cW 19.4 mm, cH 11.1 mm, dL 13.7 mm; Fig. 29A). Paratypes CUMZ 3887 (228 shells), CUMZ 3889 (53 adults + 25 juveniles; Fig. 29C), CUMZ 4704 (2 shells; Figs 29B, 32D), CUMZ 4766 (99 adults + 24 juveniles), NHMUK 20220439 (5 shells), and SMF 368674 (5 shells). All paratypes are from the type locality.

Figure 29. 

Shell of Rhiostoma ebenozostera sp. nov. A holotype CUMZ 4703 from Tham Pha Poo, Loei B, C paratypes from type locality B paratype CUMZ 4704 and C paratype CUMZ 3889 D specimen CUMZ 10211 from Lom Kao, Phetchabun E specimen CUMZ 4765 from Suwannakhuha, Nongboa Lumphu F specimen CUMZ 10166/2 from Phu Wiang, Khon Kaen.

Tham Pha Poo, Muang District, Loei Province, Thailand (17°34'45.1"N, 101°42'36.2"E). Dry dipterocarp forest on limestone hill.

Thailand: Wat Tham Pha Ban, Chiangkan, Loei: CUMZ 4444. Phu Pha Lom, Muang, Loei: CUMZ 10013. Wat Tham Phu Pha Lom, Muang, Loei: CUMZ 4443. Tham Khuha Waree, Wang Sapung, Loei: CUMZ 3925, 3946. Tham Pha Fang, Wang Sapung, Loei: CUMZ 10155. Khao Wangpha, Na Wang, Nongboa Lumphu: CUMZ 3935. Tham Suwannakhuha, Na Wang, Nongboa Lumphu: CUMZ 4765 (Fig. 29E). Wat Paphaya, Suwannakhuha, Nongboa Lumphu: CUMZ 10031. Wat Suwannakhuha, Ban Nasi, Suwannakhuha, Nongboa Lumphu: CUMZ 3889, 3933, 10021. Wat Phu Poon, Lomkao, Phetchabun: CUMZ 10211 (Fig. 29D). Tham Poo Loup, Chum Phae, Khon Kaen: CUMZ 3931. Tham Nennoi, Chum Phae, Khon Kaen: CUMZ 10015. Tham Phayanakarat, Chum Phae, Khon Kaen: CUMZ 10025. Phu Wiang, Wiang Kao, Khon Kaen: CUMZ 10166/1, 10166/2 (Fig. 29F).

Shell medium, thick and depressed. Detached whorl medium in length, curved, and slightly descending. Breathing device tubular. Broad dark brown spiral band on periphery present.

Rhiostoma ebenozostera sp. nov. can be distinguished from all other recognised Rhiostoma species by having a prominent wide dark brown to black peripheral band. This colour pattern persists in all populations recognised for this species. Regardless of the peripheral band, this new species can be distinguished from R. housei in having straight and slender tubular breathing device, and relatively longer detached whorl. In contrast, R. housei has a stout and curved tubular breathing device and relatively shorter detached whorl.

Shell. Shell medium, cW 17.7–22.6 mm, cH 9.4–12.1 mm, thick, and sub-discoidal to discoidal shape; detached-whorl length 8.5–14.5 mm. Apex acute; spire nearly flat to elevated. Whorls 4 to 5, convex, increasing regularly; suture wide and shallow; last whorl rounded and stout. Detached whorl medium in length, approximately the same length as apertural width, curved and descending. Shell surface with fine growth lines. Periostracum thin, corneous, and transparent. Shell colour uniformly brownish or with pattern of dark brown blotches, and faded on ventral shell surface; dark spiral band on periphery always present and broad. Peristome circular and double; lip thickened, expanded and multi-layered. Aperture opened sub-ventrally. Breathing device tubular, perpendicular to detached whorl and its tip attached to preceding whorl; outer lip forming long and closed tube and located just behind apertural lip; inner lip with deep incision or small hole inside aperture. Umbilicus widely opened and deep. Operculum calcareous, cup-shaped, and multispiral (Fig. 29).

Radula. Teeth arrangement and shape are similar to those of R. housei. Central tooth with large central cup and two lateral cusps on each side. Lateral teeth composed of three cusps; central cusp large, with convex tip, and flanked by small pointed tips of one outer cusp and one inner cusp. Marginal teeth each consisting of three pointed cusps (Fig. 32D).

The specific name ebenozostera is derived from the two Greek words ebenos meaning black colour, and zoste meaning girdle or belt. This name refers to the prominence of a dark, wide, peripheral band, which is the distinguishing character of this species.

This new species has narrow distribution; it is known from several limestone outcrops in dry deciduous forests of Loei, Nong Bua Lam Phu, and Khonkaen provinces (Fig. 18).

The specimens from Nong Bua Lam Phu possess a stout shell, slightly shorter detached whorl, and paler shell colour (Fig. 29E) than the typical population. However, a prominent broad, dark, peripheral band suggests that they are conspecific.

Holotype CUMZ 4500 (cW 17.1 mm, cH 8.1 mm, dL 35.0 mm; Fig. 30A). Paratypes CUMZ 3910 (40 shells), CUMZ 4350 (1 shell), CUMZ 4701 (1 shell; Figs 30B, 32E), CUMZ 10037 (8 adults + 30 juveniles), CUMZ 10038 (5 shells), NHMUK 20220440 (5 shells), and SMF 368675 (5 shells). All paratypes are from the type locality.

Ban Ping Klong (village), Chiangdao District, Chiang Mai Province, Thailand (19°30'48.6"N, 99°03'21.1"E). Small limestone hills covered by dry deciduous forest.

Thailand: Tham Sam Ta, Muang, Maehongsorn: CUMZ 4804. 1 km. from the junction to Tham Mae Ra Na, Pang Mapha, Maehongsorn: CUMZ 4440, 4464. Pang Mapha, Maehongsorn: CUMZ 4341. Tham Mae Lana, Pang Mapha, Maehongsorn: CUMZ 4702 (Fig. 30C, D), 10042, 10043 (Fig. 30E). Tham Pha Mon, Pang Mapha, Maehongsorn: CUMZ 4343. Tham Phadeang, Pang Mapha, Maehongsorn: CUMZ 10040. Wat Pa Tham Wua, Pang Mapha, Maehongsorn: CUMZ 10036. Tham Tabtao, Chai Prakarn, Chiang Mai: CUMZ 3912, 10041 (Fig. 30F). Chaiprakarn, km. 43 reach Chiang Dao, Chiang Mai: CUMZ 4738. Km. 93+200 m. Tham Klap, Pingkong, Chiang Dao, Chiang Mai: CUMZ 3908, 10039. Pa Sak Ngam, Doi Saket, Chiang Mai: CUMZ 4754. Huai Nam Dang, Mae Tang, Chiang Mai: CUMZ 10033.

Figure 30. 

Shell of Rhiostoma lannaense sp. nov. A holotype CUMZ 4500 from Pingkong, Chiang Dao, Chiang Mai B paratype CUMZ 4701 from type locality C, D paratypes CUMZ 4721 from type locality E specimen CUMZ 10043 from Pang Mapha, Maehongsorn F specimen CUMX 10041 from Chai Prakan, Chiang Mai

Shell small to medium, thin, and flattened to depressed shell. Detached whorl long, slender, curved, and descending. Breathing device tubular. Shell colour with dark brown zigzag patterns. Operculum calcareous, tall cup-shaped with loose lamellae.

Rhiostoma lannaense sp. nov. differs from R. marioni, R. thachi, and R. jalorensis in having a whitish shell with a dark brown zigzag pattern and longer detached whorl. In contrast, these three species exhibit uniformly brownish to dark brown shells and generally without a colour pattern and a relatively shorter detached whorl. In addition, R. thachi has a broader expanded aperture on the palatal side, while this new species has a thickened but not expanded lip.

This new species is superficially similar to R. tigrina sp. nov., but it can be distinguished by having very long and curved detached whorl, aperture opened ventrally, and tall cup-shaped operculum with looser lamellae. In contrast, the latter species has a shorter detached whorl, aperture opened sub-ventrally and low cup-shaped operculum with denser lamellae. Although these two species are mainly distributed in northern Thailand, the COI barcoding shows high genetic diversity (Appendix 1: Table A1) and divided R. lannaense sp. nov. as a distinct clade from other northern species, which suggests they are distinct species (Fig. 3).

Shell. Shell small to medium, cW 14.5–19.3 mm, cH 7.3–10.1 mm, thin, and nearly flattened to sub-discoidal shape; detached-whorl length 19.5–28.5 mm. Apex acute with dark colouration; spire convex to nearly flat. Whorls 4 to 5, convex, increasing regularly; suture wide and shallow; last whorl rounded and slender. Shell surface with fine growth lines. Periostracum corneous and transparent. Shell colour with brown to dark brown zigzag pattern and faded on ventral shell surface; narrow black spiral band on periphery. Detached whorl long, ~3× longer than aperture width, curved, and descending. Peristome circular and double; lip thickened, slightly expanded and multi-layered. Aperture opened sub-ventrally to ventrally. Breathing device tubular and its tip sometimes attached to preceding whorl; outer lip forming a short to long and closed tube, and located just behind apertural lip; inner lip with deep incision or small hole inside aperture. Umbilicus widely opened and deep. Operculum calcareous, tall cup-shaped, and multispiral with loose lamellae (Fig. 30).

Radula. Teeth arrangement and shape are similar to those of R. housei. Central tooth with large pointed central cusp; two lateral cusps on each side with small pointed tips. Lateral teeth consisting of four cusps; central cusp large, with dull tip, and flanked by two inner cusps and one small outer cusp. Marginal teeth each consisting of three pointed cusps (Fig. 32E).

The specific name lannaense is derived from the historical name of the Lan Na Kingdom, which flourished approximately from the 13^th to 18^th centuries. It refers to the distribution range of this new species in the northern part of Thailand, which is the approximate centre of the Lan Na Kingdom.

This new species has a narrow distribution range in a few localities in Chiang Mai and Maehongsorn provinces (Fig. 18).

There are two morphotypes occurring in this species. The typical morphotype has a long and curved detached whorl, and the tubular breathing device does not reach the preceding whorl. The shorter morphotype has a short, detached whorl, and the tubular breathing device reaches the preceding whorl (Fig. 30D). These two morphotypes are sympatric, and also have brownish zigzag colour patterns and a tall cup-shaped operculum with loose lamellae. However, the shell morphologies together with COI barcoding suggest these are conspecific.

Holotype CUMZ 4497/1 (cW 19.5 mm, cH 10.5 mm, dL 20.8 mm; Fig. 31A). Paratypes CUMZ 3909 (29 adults + 10 juveniles), CUMZ 4497/2 (2 shells; Figs 31B, 32F), CUMZ 4806 (13 adults + 5 juveniles; Fig. 31C), CUMZ 10191 (6 adults + 4 juveniles), NHMUK 20220441 (5 shells), and SMF 368676 (5 shells). All paratypes are from the type locality.

Tham Pum Tham Pla, Mae Sai District, Chiang Rai Province, Thailand (20°21'00.4"N, 99°51'25.2"E). Limestone hill covered by dry deciduous forest.

Thailand: Chiang Dao, North Thailand: ZMUC ex. B. Degerbøl collection (1 alcohol specimen at 400 m, and 2 alcohol specimens at 1100 m). Tham Samta, Muang, Maehongsorn: CUMZ 4441. Wat Pa Tham Wua, Pang Mapha, Maehongsorn: CUMZ 10028. Tham Luang, Khun Nam Nang Non, Mae Sai, Chiang Rai: CUMZ 10186, 10188. Tham Pha Chom, Mae Sai, Chiang Rai: CUMZ 4347. Tham Pla, Mae Sai, Chiang Rai: CUMZ 4346, 4803. Tham Saohin Prayanak, Mae Sai, Chiang Rai: CUMZ 3906, 4442, 10190, 10193. Wat Tham Pla School, Mae Sai, Chiang Rai: CUMZ 3913, 4844, 10189. Tham Mae Suai, Mae Suai, Chiang Rai: CUMZ 3907, 4345, 4807 (Fig. 31E, F). Khao Tham Phra, Muang, Chiang Rai: CUMZ 3915, 4795 (Fig. 31D), 4797, 4839, 4890, 10187. Wat Pha Cha Lui, Pa Dad, Chiang Rai: CUMZ 4435. Wat Tham Phra Bumpenboon, Phan, Chiang Rai: CUMZ 3914, 4495, 4496, 4706, 4846, 10027, 10034. Ban Pang Ma Yao, Chiang Dao, Chiang Mai: CUMZ 10185. Tham Chiang Dao, Chiang Dao, Chiang Mai: CUMZ 3911, 4438, 4499, 4881, 10181, 10182, 10183, 10192. Tham Brichinda, Chom Thong, Chiang Mai: CUMZ 10195. Huai Mae Muk Waterfall, Fang, Chiang Mai: CUMZ 10184.

Figure 31. 

Shell of Rhiostoma tigrina sp. nov. A holotype CUMZ 4497/1 from Tham Pum Tham Pla, Mae Sai, Chiang Rai B paratype CUMZ 4497/2 from type locality C paratype CUMZ 4806 from type locality D specimen CUMZ 4795 from Tham Phra, Chiang Rai E, F specimens CUMZ 4807 Mae Suai, Chiang Rai

Shell small to medium, slightly thin and depressed. Detached whorl medium in length, curved and descending. Breathing device tubular and usually attached to preceding whorl. Shell colour with dark brown zigzag patterns on both sides.

This new species differs from R. marioni, R. jalorensis, and R. thachi in having a brown zigzag colour pattern and short detached whorl. In contrast, these three species have uniformly brownish to dark brown shells (rarely with a blotched pattern) and long and twisted detached whorls. In addition, R. thachi has a broader expanded aperture on the palatal side, while R. trigrina sp. nov. has a thickened, slightly expanded, multi-layered lip.

Shell. Shell small to medium, cW 15.7–20.5 mm, cH 8.5–11.7 mm, slightly thin, and sub-discoidal to discoidal; detached-whorl length 4.5–13.5 mm. Apex acute and dark; spire convex to nearly flat. Whorls 4 to 5, convex, increasing regularly; suture wide and shallow; last whorl rounded and stout. Shell surface with fine growth lines. Periostracum thin, corneous, and transparent. Shell with brown to dark brown zigzag patterns and faded on ventral shell surface, with narrow, dark, spiral band on periphery. Detached whorl medium in length, approximately the same length as aperture width, curved and descending. Peristome circular and double; lip thickened, expanded and multi-layered. Aperture opened sub-ventrally. Breathing device tubular and curved or straight, and its tip usually attached to preceding whorl; outer lip forming a long and closed tube, and located just behind apertural lip; inner lip with hole inside aperture. Umbilicus widely opened and deep. Operculum calcareous, cup-shaped, and multispiral (Fig. 31).

Radula. Teeth arrangement and shape are similar to those of R. housei. Central tooth with triangular central cusp, flanked by two lateral cusps on each side, each pair with pointed tips. Lateral teeth composed of four cusps; central cusp large with dull tip and flanked by one outer and two inner cusps. Marginal teeth each consisting of three pointed cusps (Fig. 32F).

Figure 32. 

Radula morphology A Rhiostoma housei specimen CUMZ 3982 from Wat Tham Srivilai, Saraburi B Rhiostoma hainesi specimen from Chanthaburi C Rhiostoma jalorensis specimen CUMZ 3994 from Wat Tham Sua, Krabi D Rhiostoma ebenozostera sp. nov. paratype CUMZ 4704 from Tham Pha Poo, Loei E Rhiostoma lannaense sp. nov. paratype CUMZ 4701 from Chiang Dao, Chiang Mai F Rhiostoma tigrina sp. nov. paratype CUMZ 4497/2 from Mae Sai, Chiang Rai.

The specific name tigrina comes from the Latin word tigris. It refers to the brownish streak or zigzag colour pattern on the shell surface of this new species, which is similar to the colour pattern of the Bengal tiger, Panthera tigris tigris (Linnaeus, 1758).

This species is mainly distributed in northern Thailand in Chiang Rai, Maehongsorn, and Chiang Mai provinces (Fig. 18).

This species has high variation in shell colour, from dark brown zigzag patterns to whitish shells without patterns (Fig. 31F) among specimens from the same collection localities.

Lectotype (designation in Zilch (1956)) SMF 130509/1 (Fig. 35A) from Golf von Siam: Insel Samui. Paralectotypes SMF 130510/5 (5 shells), SMF 130511/2 (2 shells), SMF 130512/1 (1 shell).

Thailand: Siam: NHMW 6657 ex. Gerstenbrandt collection (1 shell). Insel Samui, Golf von Siam: NHMW 40488 ex. Möllendorff collection (1 shell). Koh-Samui, Gulf of Siam: NHMW Rüsnov collection (2 shells). Koh Samsao, Angthong Islands, Koh Samui, Surat Thani: CUMZ 3839, 4756. Koh Wua Ta Lab, Angthong Islands, Koh Samui, Surat Thani: CUMZ 3872, 4365, 4400, 4715, 4730, 4767 (Fig. 35D–F), 4825, 4826 (Figs 33, 35B–C, 42A).

Figure 33. 

Live snails of the species group III. Rhiostoma asiphon specimen CUMZ 4826 from Koh Wua Ta Lab, Ang Angthong, Surat Thani (topotype), A life habitus and B fully extending body underwater. All figures are not to scale. Photographs: P. Tongkerd

Shell thick and depressed; detached whorl short. Shell uniformly brown to purplish, sometimes with yellowish colour. Breathing device notch-shaped.

Rhiostoma aiphon is superficially similar to R. samuiense in having a uniformly brownish to purplish shell and notch-shaped breathing device. However, this species can be distinguished by having an extremely short detached whorl, while R. samuiense possesses a medium to long and descending detached whorl.

Shell. Shell small to medium, cW 18.4–22.7 mm, cH 9.4–12.5 mm, thickened, and sub-discoidal shape; detached-whorl length 2.5–8.0 mm. Apex acute, earlier whorls with dark colouration; spire nearly flat. Whorls 4 to 5, convex, increasing regularly; suture wide and very shallow; last whorl rounded and stout. Shell surface with fine growth lines. Periostracum brownish to thin, corneous, and translucent. Shell colour uniformly brown to purplish; with thin and narrow brownish peripheral band sometimes present. Detached whorl shorter than apertural width to nearly absent. Peristome circular and double; lip thickened, expanded, and multi-layered. Aperture opened sub-laterally. Breathing device notch-shaped; outer lip protruding, with shallow notch; inner lip with shallow incision to indistinct. Umbilicus widely opened and deep. Operculum calcareous, cup-shaped, and multispiral (Fig. 35).

Radula. Taenioglossate radula arranged in inverted V-shaped rows. Central tooth with well-developed central cusp and three lateral cusps on each side; central cusp large and tall with blunt tip; lateral cusps triangular, tapering in size and with pointed tips. Lateral teeth consisting of four cusps; central cusp large, blunt tip, and flanked by pointed tips of two inner cusps and one outer cusp. Inner and outer marginal teeth each composed of three cusps; central cusp large with dull tip, flanked by small pointed tips of one inner cusp and one outer cusp (Fig. 42A).

The previous record of this species was only from the type locality “Insel Samui, Golf von Siam” (Möllendorff 1893, 1894). The records of “R. asiphon” on the Malay Peninsula (Thailand and Malaysia) are based on misidentifications and excluded herein. Therefore, this species is probably endemic to the Ang Thong Islands in Surat Thani Province. These isolated limestone islands are located ~30 km to the west of Samui Island, Surat Thani Province, and this is considered the type locality of this species (Fig. 34).

Figure 34. 

Distribution map of Rhiostoma asiphon–species group.

Under the name Rhiostoma asiphon [non Möllendorff 1893], this species has been reported from several places on the Malay Peninsula (see Benthem Jutting 1960; Berry 1963; Chan 1996) and also reported as “Pterocyclus [sic] asiphon” from several localities in southernmost Thailand (Tumpeesuwan 2001). We examined these specimens and revisited all mentioned localities, but found no samples that could be provisionally identified as R. asiphon s. s. On the other hand, numerous specimens of Pterocyclos diluvium, common snails in those localities, were collected (see Sutcharit et al. 2014). Their shell morphology is superficially similar to R. asiphon. However, the distinguishing characters of the genus Pterocyclos are an outer lip expanded at the base, forming a nearly closed tube; an apertural groove is located inside the aperture near the apertural lip; and a calcified plate-shaped operculum is present (Fig. 2). Therefore, we conclude that the nominal species name R. asiphon in Benthem Jutting (1960), Berry (1963), and Chan (1996) and Pterocyclus asiphon in Tumpeesuwan (2001) and Tumpeesuwan and Panha (2003) are erroneously identified and should be recognised as Pterocyclos diluvium.

Of the paralectotypes SMF 130510/5 (5 shells), four of them are identical to the lectotype specimen (Fig. 35A). The exception is a single specimen exhibiting canal-shaped breathing device that is expanded at the base near the suture, similar to the distinct characters of either R. morleti or R. prestoni. However, this is beyond the range of individual variation, and the recent topotype specimens (~100 shells) included no shells exhibiting these peculiar characters. Therefore, we excluded that specimen from our attention in the type series.

Figure 35. 

Shell of Rhiostoma asiphon A lectotype SMF 130509 from Insel Samui, Golf von Siam B, C specimens CUMZ 4826 from Koh Wua Ta Lab, Angthong Islands, Surat Thani with dark shell form D–F specimens CUMZ 4767 from Koh Wua Ta Lab, Angthong Islands, Surat Thani with yellowish shell form.

The original description did not explicitly indicate the number of specimens, and was without illustration or explicit designation of the name-bearing type, although one set of measurements was given in Möllendorff (1899). Later, Kobelt (1911: pl. 113, fig. 11) illustrated this species for the first time, and then Zilch (1956) recognised this figured specimen (SMF 130513) as the “holotypus”. This SMF collection lot contains only one specimen; therefore, the subsequent use of the term “holotypus” in Zilch (1956) constitutes a valid lectotype designation. Lectotype (designation in Zilch (1956)) SMF 130513 (Fig. 36A) from S-Shan Staaten: Kalow, 5000’.

Figure 36. 

Shell of Rhiostoma strubelli A lectotype SMF 130513 from S-Shan Staaten: Kalow B–E specimens CUMZ 10200 from Vieng Phuka, Luang Namtha, Laos and E with long breathing device F specimen CUMZ 10047 from Xishuangbanna Botanical Garden, Yunnan, China.

China: Xishuangbanna Tropical Botanical Garden, Mengla County, Xishuangbanna Prefecture, Yunnan Province: CUMZ 10047 (7 shells; Fig. 36F). Laos: Khao Pa Huak, Vieng Phuka District, Luang Nam Tha Province: CUMZ 10200 (29 shells; Figs 36B–E, 42B).

Shell small, thin, and depressed; detached whorl shorter than apertural width. Shell with brownish zigzag patterns. Breathing device short and incomplete tube shape.

Rhiostoma strubelli can be distinguished from R. haughtoni and R. tigrina sp. nov. by having a smaller shell, very short detached whorl, incomplete tubular shape of breathing device, and apex with dark colouration. In contrast, R. haughtoni has a canal-shaped breathing device with a monochrome shell, while R. tigrina sp. nov. has a long, detached whorl and short to long breathing device with complete tube shape.