This species was described from specimens from the H. Cuming collection. The number of specimens was not indicated, but only one set of measurements was given in the original description. Only one specimen from the H. Cuming collection at NHMUK has Pfeiffer’s handwriting on the species name label. It is identical to the illustration and measurements in Pfeiffer (1854: 394, 395, pl. 145, figs 15–17) and is designated here as the lectotype to stabilize the name: NHMUK 20130684 (Fig. 4A).

Lectotype of Streptaxis paradiscus Möllendorff, 1900 SMF 108534 (Fig. 4B) and paralectotypes SMF 108535 (5 shells). Marble Mountain, Da Nang, Vietnam (16°0'13.4"N, 108°15'49.1"E): CUMZ 6001 (39 shells; Fig. 4C), 6257 (6 specimens in ethanol; Figs 11A, B, 22A). Annam: MNHN Jousseaume Coll. (1 shells), MNHN Denis Coll. (2 shells), MNHN Letellier Coll. (3 shells), NHMW 40858 (2 shells), NHMUK 1901.12.23.13–14 (2 shells), NHMUK Trechmann coll. Acc. 2176 (2 shells), NHMUK Connolly Coll. Acc. 2154 (1 shell), RMNH Fruhstorfer Coll. 45a (1 shell), RMNH Saverbgen Coll. (2 shells). Tourane [=Da Nang], Central Annam: NHMUK McAndrew coll. Acc. 1563 (2 shells). Touraine, Annam: NHMW Rusnov Coll. R 283 (2 shells), NMW 1955.158.25251 (1 shell), RMNH Verdcourt Coll. (2 shells), ZMB 6619 (3 shells), ZMB 52300 (3 shells). Phuc-Son, Annam: NHMW 31140 (1 shell).

Shell. Shell flattened, white and translucent; whorls 6–6½; spire flattened with distinct suture. Shell surface glossy, smooth with growth lines and varices present. Embryonic shell large, about 2½ whorls, with a smooth surface; following whorls regularly coiled. Last whorl angular, regularly expanded; umbilicus very wide, shallow and showing all preceding whorls. Aperture semi-ovate; peristome discontinuous, thickened, expanded and reflected; apertural dentition with only one parietal lamella (Fig. 4A–C).

Radula. Each row consists of 61–67 teeth with formula (30-33)-1-(30-33). Central tooth very small and triangular with pointed cusp. Lateral and marginal teeth undifferentiated, unicuspid and lanceolate. Latero-marginal teeth gradually reduce in size, with outermost teeth smaller and shorter than inner teeth (Fig. 22A).

Genital organs. Atrium (at) short; penis (p) long and slender. Penial sheath (ps) thin, extending about half to third-fourths of penis length; penial sheath retractor muscle (psr) very thin, originating at atrium and inserting at distal end of penial sheath (Fig. 11A). Vas deferens (vd) passes through a very short part of penial sheath before entering into penis distally (Fig. 11B). Penial retractor muscle (pr) thin and very long, inserting at penis and vas deferens junction.

Vagina (v) long, cylindrical, about two thirds of penis length. Gametolytic duct (gd) a long tube extending as far as albumin gland; gametolytic sac (gs) ovate. Free oviduct (fo) short; oviduct (ov) enlarged and folded. Prostate gland inconspicuous and bound to oviduct. Talon (ta) small. Hermaphroditic duct (hd) bearing long seminal vesicle (sv) about one and half times longer than the length from talon to branching point of seminal vesicle (Fig. 11A).

Pallial system. Excretory system typically sigmurethran and without mantle gland. Heart (h, auricles and ventricle) located left of kidney (on right in Fig. 11C). Pulmonary cavity approximately three times longer than broad. Pulmonary vein (puv) and venation on lung roof distinct and well developed. Kidney (k) very short, located at posterior of pulmonary cavity. Ureter (ur) sigmoid, closed tube arising from apex of kidney, extending along right side of kidney, recurving adjacent to rectum (rt). Anus (a) adjacent to pneumostome (pn) on mantle collar.

The type specimen discovered in the H. Cuming collection at NHMUK elucidates two issues. Firstly, Streptaxis paradiscus Möllendorff, 1900 has been recognized as a separate species in many works (Möllendorff 1900, Gude 1903, Kobelt 1906, Benthem Jutting 1954, Zilch 1960, 1961, Schileyko 2011). However, based on the type specimens, Discartemon discus and Streptaxis paradiscus are identical in all shell characters. Therefore, we officially place Streptaxis paradiscus as a junior subjective synonym of Discartemon discus. Second, Discartemon discus had an unknown type locality and range (Pfeiffer 1851: 252). From the new material and the type locality of Streptaxis paradiscus, the distribution of this species is demarcated to several localities in the area of Da Nang, Vietnam (Schileyko 2011).

The record of Streptaxis discus from Brazil, mentioned in Bourguignat (1899) is almost certainly an error, since it is far beyond the distribution range of the genus. The specimen figured in Simone (2006: 191, fig. 708, reg. NHMUK Trechmann Acc. 2176) has the locality Annam [= central Vietnam].

Celebes [=Sulawesi], Indonesia: NMW 1955.158.25252 (1 shell; Fig. 4D).

The shell of this species is clearly distinct from all other recognized species. Shell flattened, with a concave spire and distinct suture. Shell surface smooth, varices present; whorls regularly coiled. Last whorl rounded with keel below periphery, rapidly expanded; umbilicus very wide, concave and showing all preceding whorls. Aperture triangular with long and narrow sinulus, peristome thickened and little reflected. Apertural dentition with one parietal, one palatal and one columellar lamella (Fig. 4D) (Fulton 1899, Kobelt 1906, Marwoto 2008).

The distribution of Discartemon planus seems to be outside the ranges of all other Discartemon species, and is probably restricted to the limestone karst in the south of Sulawesi (Fulton 1899, Sarasin and Sarasin 1899, Laidlaw 1929, Bruggen 1972, Marwoto 2008). It does not closely resemble any other streptaxid genus more closely than Discartemon. However, the very wide umbilicus showing all preceding whorls and surrounded with a keel, with a long and narrow adapical sinulus, may indicate that Discartemon planus comprises a distinct lineage within Discartemon. Both Bruggen (1972) and Marwoto (2008) discussed the possibility that it required a separate genus or subgenus, but anatomical or molecular evidence are desirable to support this assertion.

Paratypes NHMUK 1937.7.9.11 (1 shell; Fig. 4F) and NMW 1955.158.25257 (1 shell; Fig. 4E).

The distinguishing characters of this species are the flattened shell and spire with a distinct suture. Shell surface nearly smooth with thin growth lines, varices present; following whorls regularly coiled. Last whorl angular, intermediately expanded; umbilicus very wide and showing all preceding whorls. Aperture semi-ovate with sinulus; peristome thickened, expanded and reflected; apertural dentition with only one parietal lamella (Fig. 4E).

Discartemon sykesi differs from Discartemon discus in its smaller shell, in the presence of a sinulus, the intermediately expanded last whorl, and in being restricted to the Malay Peninsula. This species can be distinguished from Discartemon planus in having a larger shell with flattened spire, the last whorl angular and intermediately expanded, a semi-ovate aperture, and in lacking palatal and columellar lamellae.

Khao Ok Thalu, Phatthalung, Thailand (7°37'39.1"N, 100°5'19.1"E): CUMZ 3594 (24 shells; Fig. 4G) and 6208 (12 specimens in ethanol; Figs 2A, 11D, E, 17A–E, 22B).

Shell. Shell flattened, white and semi-transparent; whorls 5½, spire flattened with distinct suture. Shell surface glossy with thin transverse ridges near suture and varices present. Embryonic shell about 2½ whorls; following whorls regularly coiled. Last whorl angular with strong peripheral keel, regularly expanded; umbilicus very wide and showing all preceding whorls. Aperture triangular with sinulus; peristome continuous, thickened, expanded and reflected. Apertural dentition with a Y-shaped parietal lamella adjoining at sinulus (Fig. 4G).

Radula. Each row consists of 39–41 teeth with formula (19-20)-1-(19-20). Central tooth very small and triangular with pointed cusp. Lateral and marginal teeth undifferentiated, unicuspid and lanceolate. Latero-marginal teeth gradually reduce in size, with outermost teeth smaller and shorter than inner teeth (Fig. 22B).

Genital organs. Atrium (at) long. Proximal penis (p) long, slender; distal penis globularly enlarged. Penial sheath (ps) thin, extending about two-thirds of penis length; penial sheath retractor muscle very thin (psr), originating at genital orifice wall and inserting distally on penial sheath (Fig. 11D). Vas deferens (vd) passes through about one-fifth of penial sheath length before entering into penis distally (Fig. 11E). Penial retractor muscle (pr) thin and very long, inserting at penis and vas deferens junction.

Internal wall of atrium generally smooth (Fig. 17A); penial wall with scattered, short and transparent penial hooks, about 5 hooks/200 µm² (Fig. 17B); hooks located on round-ovate penial papilla. Penial hooks of small size (<0.04 mm in length), slightly expanded at base, tip obtuse and directed towards genital orifice (Fig. 17C, D).

Vagina (v) short and stout, about half of penis length. Gametolytic duct (gd) a long tube extending as far as albumin gland; gametolytic sac (gs) ovate. Free oviduct (fo) long and thick; oviduct (ov) enlarged and folded; prostate gland inconspicuous and bound to oviduct. Talon (ta) small, very short and club shaped. Hermaphroditic duct (hd) bearing long seminal vesicle (sv) about one and half times longer than the length from talon to branching point of seminal vesicle (Fig. 11D).

Vaginal wall generally with smooth surface of longitudinal vaginal folds (Fig. 17E).

Discartemon nummus was described from Tale Sap (= Lake or Lagoon), Singgora (= Songkhla). In this study, living snails were found at an isolated limestone hill near the lake in Phatthalung, about 60 km north of the type locality.

Having the smallest shell size clearly discriminates Discartemon nummus from all congeners. It is similar to Discartemon discus, which has a larger shell and a peripheral keel, lacks a sinulus, and has a semi-ovate aperture with a straight parietal lamella. Discartemon nummus can be distinguished from Discartemon khaosokensis in having a flattened spire, the last whorl regularly expanded, a triangular aperture, and a Y-shaped parietal lamella.

Holotype CUMZ 6242 (Fig. 4H). Measurement: shell height 3.6 mm, shell width 11.4 mm, and with 5¾ whorls. Paratype CUMZ 6243 (4 shells).

This species is known only from the type locality. The shell is flattened and semi-transparent and has a concave spire with a distinct suture. Shell surface with transverse ridges that diminish below periphery, with varices present; whorls regularly coiled. Last whorl angular with a strong peripheral keel, rapidly expanded; umbilicus very wide, showing all preceding whorls. Aperture semi-ovate with narrow sinulus; peristome thin and expanded; apertural dentition of only one parietal lamella (Fig. 4H).

Discartemon khaosokensis differs from Discartemon discus in having a smaller shell, concave spire, a shell surface with tranverse ridges, a rapidly expanded last whorl with a strong peripheral keel, and a sinulus. Discartemon khaosokensis is also similar to Discartemon sykesi, but has a larger shell, a concave spire with tranverse ridges, and a rapidly expanded last whorl with a strong peripheral keel.

Holotype CUMZ 6244 (Fig. 4I). Measurement: shell height 4.5 mm, shell width 12.1 mm, and with 6 whorls. Paratypes: CUMZ 6003 (2 shells), 6209 (1 specimen in ethanol; Figs 2B, 12A, B, 17F–I), 6258 (4 shells), NHMUK 20130672 (1 shell), and SMF (1 shell) from the type locality.

Wat Tam Yai, Thachana, Suratthani, Thailand (9°32'21.5"N, 99°11'29.4"E).

This new species can be distinguished from Discartemon discus and Discartemon sykesi by having transverse ridges that diminish below the periphery, and having an apertural dentition with five lamellae. In comparison, Discartemon sykesi has a smaller shell and Discartemon discus has a higher spire. The genitalia of Discartemon discus have a short penis, penial sheath and free oviduct, and long vagina while Discartemon discadentus sp. n. has a very long penis, penial sheath and free oviduct, and short vagina. Discartemon discadentus sp. n. differs from Discartemon nummus and Discartemon khaosokensis in having a larger shell with higher spire, in lacking a peripheral keel, and in usually having five apertural lamellae. The last whorl of Discartemon khaosokensis is rapidly expanded, while Discartemon nummus has a regularly expanded last whorl and Y-shaped parietal lamella. The genitalia of Discartemon discadentus sp. n. differ from those of Discartemon nummus in the long and slender penis, penial wall with reticulated folds, and long penial hooks located on conical penial papillae.

Shell. Shell flattened, white and translucent; whorls 6; spire only slightly elevated; suture distinct. Shell surface glossy with transverse ridges that diminish below periphery; varices present. Embryonic shell large, about 2½ whorls, with smooth surface; following whorls regularly coiled. Last whorl angular, intermediately expanded; umbilicus very wide and showing all preceding whorls. Aperture semi-ovate; peristome discontinuous, thickened and expanded. Apertural dentition usually with one strong parietal, one palatal, one small basal and one strong columellar lamella. A small supracolumellar lamella is sometimes present (Fig. 4I).

Genital organs. Atrium (at) short; penis (p) very long and slender. Penial sheath (ps) thin, extending about five-sixths of penis length; penial sheath retractor muscle very thin (psr), originating at genital orifice wall and inserting distally on penial sheath (Fig. 12A). Vas deferens (vd) passes through a very short part of penial sheath before entering into penis distally (Fig. 12B). Penial retractor muscle (pr) thin and very long, inserting at penis and vas deferens junction.

Internal wall of atrium generally corrugated (Fig. 17F). Penial wall with scattered and transparent penial hooks, about 6 hooks/200 µm² (Fig. 17G); hooks located on conical penial papillae (pp) separated by low reticulated folds. Penial hooks small (<0.03 mm in length), expanded at base, tips pointed and curved towards genital orifice (Fig. 17H).

Vagina (v) short, about one seventh of penis length. Gametolytic duct (gd) a long tube extending as far as albumin gland; gametolytic sac (gs) ovate. Proximal free oviduct (fo) convoluted and distally long and thick; oviduct (ov) enlarged and folded. Prostate gland inconspicuous and bound to oviduct. Talon (ta) small, short and club shaped. Hermaphroditic duct (hd) bearing long seminal vesicle (sv) about four times longer than the length from talon to branching point of seminal vesicle (Fig. 12A).

Vaginal wall generally with smooth surface of reticulated vaginal folds (Fig. 17I).

The specific epithet “discadentus” is derived from the Latin “discus” meaning “disc” and “dentatus” meaning “teeth”.

This species seems to be restricted to limestone areas in Suratthani Province, Thailand. Tam Khuha, Kanchanadit District, is an isolated limestone hill about 40 km southeast of the type locality and Wat Tam Wararam, Phanom District, is in the limestone mountains near Ratchaprapa Dam, about 70 km southwest of the type locality.

This species shows variation in shell size and the presence of the infrapalatal, upper palatal and supracolumellar lamellae. Some specimens from Phanom, Suratthani (CUMZ 3571, 3582) possess an upper palatal and supracolumellar lamella, and an infrapalatal lamella is present in one paratype shell (CUMZ 6003). Populations from Tam Khuha, Suratthani (CUMZ 6004) exhibit a relatively smaller shell size (width about 11 mm). This new species is apparently rare and only extensive searching revealed living animals.

Holotype CUMZ 6245 (Fig. 5A). Measurement: shell height 4.7 mm, shell width 14.6 mm, and with 7 whorls. Paratypes: CUMZ 6005 (2 shells) and NHMUK 20130673 (2 shells) from the type locality.

Tam Kobe, Phangnga: CUMZ 3669, 6197.

Tam Namphud, Phangnga, Thailand, 8°27'46.8"N, 98°32'30.5"E.

The characters distinguishing Discartemon discamaximus sp. n. from Discartemon sykesi and Discartemon khaosokensis are the larger shell with flattened to concave spire, the transverse ridges present near the suture, and the lack of a sinulus. Discartemon discamaximus sp. n. has similar shell morphology to Discartemon discus and Discartemon discadentus sp. n., but is distinguished by having the transverse ridges present only near the suture and the last whorl rapidly expanded. Discartemon discadentus sp. n. also has five apertural lamellae.

Shell. Shell flattened, white and translucent; whorls 7, spire flattened to concave, with distinct suture. Shell surface glossy with transverse ridges near suture and varices present. Embryonic shell large, about 2½ whorls, with smooth surface; following whorls regularly coiled. Last whorl angular, rapidly expanded; umbilicus very wide and showing all preceding whorls. Aperture semi-ovate; peristome discontinuous, expanded and reflected; apertural dentition with one parietal lamella (Fig. 5A).

The specific epithet “discamaximus” is derived from the Latin “discus” meaning “disc” and “maximus” meaning “large or broad”.

This new species is known from limestone karst near Phanganga Bay reaching about 100–400 meters amsl, surrounded by the Phuket mountain range.

To date no living specimens have been found.

Holotype CUMZ 6246 (Fig. 5C). Measurement: shell height 3.9 mm, shell width 7.7 mm, and with 6 whorls. Paratypes: CUMZ 3665 (9 shells), 6262 (8 shells), NHMUK 20130674 (2 shells), and SMF (2 shells) from the type locality.

Tam Phannara, Nakhon Si Thammarat, Thailand, 8°25'18.8"N, 99°22'46.8"E.

Discartemon circulus sp. n. differs from Discartemon discus and Discartemon sykesi in its narrower umbilicus, sub-quadrangular aperture, and apertural dentition with five lamellae. In addition, Discartemon discus has a larger shell, while Discartemon sykesi has an intermediately expanded last whorl and a sinulus. Compared with Discartemon khaosokensis, Discartemon circulus sp. n. has a smaller shell, a flattened spire with weak transverse ridges, an angular last whorl, a sinulus, and five apertural lamellae. Discartemon circulus sp. n. differs from Discartemon discadentus sp. n. and Discartemon discamaximus sp. n. in having a smaller shell with weak transverse ridges and five apertural lamellae. Compared with Discartemon expandus sp. n., Discartemon circulus sp. n. has weaker transverse ridges, a regularly expanded peristome, and five apertural lamellae.

Shell. Shell flattened, white and translucent; whorls 5½–6, spire flattened, with a distinct suture. Shell surface glossy with weak transverse ridges and varices present. Embryonic shell large, about 2½ whorls, with a smooth surface; following whorls regularly coiled. Last whorl angular, regularly expanded; umbilicus very wide, deep and showing all preceding whorls. Aperture sub-quadrangular; peristome discontinuous, thin and expanded. Apertural dentition with one parietal, one palatal, one small basal, one small columellar and one small supracolumellar lamella (Fig. 5C).

The specific epithet is from the Latin “circulus” meaning “circle”. It refers to the appearance of this new species when seen from the apex.

This species is known only from the type locality, an isolated limestone hill which reaches about 200 meters amsl, about 20 km southwest of Tai Rom Yen National Park.

Apparently rare and extensive searching revealed no living examples.

Holotype CUMZ 6247 (Fig. 5E). Measurement: shell height 2.5 mm, shell width 8.2 mm, and with 5 whorls. Paratypes: CUMZ 3573 (2 shells), NHMUK 2013675 (1 shell), and SMF (1 shell) from the type locality. Paratype: CUMZ 6259 from Ban Tam Thong, Prathiew, Chumphon.

Khao Pu-Khao Ya National Park, Sri Banphot, Phatthalung: CUMZ 3670.

Khao Hup Ta Hae, Prathiew, Chumphon, Thailand, 10°48'44.9"N, 99°25'9.0"E.

This species closely resembles Discartemon sykesi, but is distinct in having a concave spire and strong peripheral keel on the last whorl. Compared with Discartemon khaosokensis, Discartemon deprima sp. n. has a smaller shell with weaker transverse ridges, and the last whorl intermediately expanded. Discartemon deprima sp. n. differs from Discartemon nummus by having a larger shell, a concave spire, and one straight parietal lamella. It differs from Discartemon circulus sp. n. and Discartemon expandus sp. n. in having a concave spire, the last whorl intermediately expanded with a strong peripheral keel, and in having only one parietal lamella and a sinulus. In addition, Discartemon expandus sp. n. has transverse ridges that diminish below the periphery, and has a thin and widely expanded peristome.

Shell. Shell flattened, white and semi-transparent; whorls 5, spire concave with a distinct suture. Shell surface glossy with weak transverse ridges that diminish below periphery and appear again near peristome; varices present. Embryonic shell large, about 2½ whorls, with a smooth surface; following whorls regularly coiled. Last whorl angular with strong peripheral keel, intermediately expanded. Umbilicus very wide and showing all preceding whorls. Aperture semi-ovate with sinulus; peristome discontinuous, thin, expanded and reflected. Apertural dentition with only one parietal lamella (Fig. 5E).

The specific epithet “deprima” is derived from the Latin “deprimo” meaning “depress”. It refers to the depressed spire of this new species.

This species is known from the east coast of Chumphon, on an isolated limestone hill reaching about 200 meters amsl, and from a more southerly locality in Patthalung, a limestone hill complex reaching about 200–400 meters amsl.

There is some variation in this species in the discontinuous peristome and the presence of a sinulus. The samples from Patthalung (CUMZ 3670, 2 shells) have a continuous peristome and lack a sinulus. Currently, no living examples have been found.

Holotype CUMZ 6248 (Fig. 5G). Measurement: shell height 3.8 mm, shell width 8.3 mm, and with 5½ whorls. Paratypes: CUMZ 3664 (10 shells), NHMUK 20130676 (2 shells), and SMF (2 shells) from the type locality.

Klong Hoy, Suratthani, Thailand, 8°57'18.1"N, 98°48'30.7"E.

Discartemon expandus sp. n. differs from Discartemon discus and Discartemon sykesi in its smaller shell with transverse ridges, intermediately expanded last whorl, and widely expanded peristome. In addition, a sinulus is absent in Discartemon discus. Discartemon expandus sp. n. can be distinguished from Discartemon khaosokensis by having a flattened spire, an angular and intermediately expanded last whorl, and a widely expanded peristome. Compared with Discartemon discadentus sp. n. and Discartemon discamaximus sp. n., Discartemon expandus sp. n. has a smaller shell with strong transverse ridges, a sinulus and a widely expanded peristome. Moreover, Discartemon discadentus sp. n. has five apertural lamellae, and Discartemon discamaximus sp. n. has a rapidly expanded last whorl.

Shell. Shell flattened, white and semi-transparent; whorls 5½–6, spire flattened with a distinct suture. Shell surface glossy with transverse ridges that diminish below periphery; varices present. Embryonic shell large, about 2–2½ whorls, with a smooth surface; following whorls regularly coiled. Last whorl angular, intermediately expanded; umbilicus very wide, deep and showing all preceding whorls. Aperture semi-ovate with narrow sinulus; peristome discontinuous, thin and widely expanded. Apertural dentition of only one parietal lamella (Fig. 5G).

The specific epithet “expandus” is derived from the Latin “expandi” meaning “expand”. It refers to the expanded peristome of this species.

The species is known only from the type locality and extensive searching revealed no living examples.

Some variation has been observed in the spire, which is slightly convex rather than flattened in some specimens, and in the distinctness of the suture.

Perak, Malaysia NHMUK 1939.4.13.22 (1 shell; Fig. 6A). Sungei Siput, Perak, Malaysia: NMW 1955.158.25253 (13 shells). RMNH Kaumans Reg. 598 (3 shells). Hot Springs, Tanjung Rambutan, Perak, Mlaysia: RMNH Drijver Coll. (1 shell). Yan Tao San, Perak, Malaysia: CUMZ 6008. Ipoh, Perak, Malaysia (4°36'34.6"N, 101°6'49.9"E): CUMZ 6009 (Fig. 6B; 3 shells).

The original descriptions included informative figures (Morgan 1885a, b), and subsequently Benthem Jutting (1954: fig. 2) published excellent figures of topotype specimens. These allow unambiguous recognition of this species. Shell depressed-heliciform with a flattened spire. Shell surface with transverse ridges that diminish below periphery and varices present; following whorls regularly coiled. Last whorl rounded and regularly expanded; umbilicus widely open and deep. Aperture triangular with sinulus, and apertural dentition with one parietal and one palatal lamella (Fig. 6B).

Compared with Discartemon leptoglyphus and Discartemon plussensis, it differs in its smaller shell, with transverse ridges that diminish below periphery, last whorl rounded and more inflated, and sinulus present.

Holotype ZMA 3.34.017 (Fig. 6C). Paratypes: ZMA 3.54.018 (5 shells). Bukit Chuping, Perlis, Malaysia (6°29'36.2"N, 100°15'53.2"E): CUMZ 6011 (2 shells; Fig. 6D). Guplu Bukit, Perlis, Malaysia: CUMZ 6198. Kaki Bukit, Perlis, Malaysia: CUMZ 6199 (1 specimen in ethanol; Figs 2D, 12C, D, 18A–E, 22C).

Shell. Shell depressed-heliciform, white and semi-transparent; whorls 5–5½, spire only slightly convex with distinct suture. Shell surface glossy with thin transverse ridges that diminish below periphery; varices present. Whorls regularly coiled; last whorl rounded, regularly expanded; umbilicus very wide, deep and showing all preceding whorls. Aperture triangular; peristome discontinuous, thickened, expanded and reflected. Apertural dentition with one sinuous parietal, one palatal, one columellar and one supracolumellar lamella (Fig. 6C).

Radula. Each row consists of 43 teeth with formula (21)-1-(21). Central tooth very small and triangular with a pointed cusp. Lateral and marginal teeth undifferentiated, unicuspid and lanceolate. Latero-marginal teeth gradually reduce in size, with outermost teeth much smaller and shorter than inner teeth (Fig. 22C).

Genital organs. Atrium (at) long and thick. Proximal penis (p) with short and stout penial appendix (pa) about two-thirds of penis length; distal penis slender (Fig. 12C). Penial sheath retractor muscle (psr) very thin, originating at genital orifice wall and inserting distally on penial sheath (Fig. 12C). Vas deferens (vd) passes through about a quarter of penial sheath length before entering into penis distally (Fig. 12D). Penial retractor muscle (pr) thin and very long, inserting at penis and vas deferens junction.

Internal wall of atrium generally smooth with numerous atrial pores (Fig. 18A). Penial wall with dense and brownish penial hooks, about 4 hooks/200 µm² (Fig. 18B). Hooks located on laterally-flattened penial papillae (pp), which are separated by thin reticulated folds. Penial hooks very small (< 0.01 mm in length), expanded at base, pointed at tip and curved towards genital orifice (Fig. 18C, D).

Vagina (v) short. Gametolytic duct (gd) enlarged and stout at base, and suddenly tapering to small and long tube extending as far as albumin gland; gametolytic sac (gs) ovate. Free oviduct (fo) proximally large with equivalent diameter to vagina, tapering to smaller tube distally. Oviduct (ov) enlarged and folded; prostate gland inconspicuous and bound to oviduct. Talon (ta) small, short and club shaped. Hermaphroditic duct (hd) bearing long and thick seminal vesicle (sv) about one and half times longer than the length from talon to branching point of seminal vesicle (Fig. 12C).

Vaginal wall with reticulated vaginal folds (Fig. 18E).

This species is known from several limestone hills in Perlis, Malaysia.

Discartemon hypocrites can be distinguished from Discartemon plussensis, Discartemon leptoglyphus and Discartemon platymorphus by the apertural dentition with one sinuous parietal, one columellar, and one supracolumellar lamella. The latter three species exhibit one straight parietal and one palatal lamella. In addition, Discartemon plussensis has a lower spire, an inflated last whorl and a sinulus; Discartemon leptoglyphus has transverse ridges over the entire shell; and Discartemon platymorphus has a larger shell and lower spire. Discartemon hypocrites also differs from Discartemon leptoglyphus in having a slender penis with short and stout penial appendix, the vas deferens passing through about a quarter of penial sheath length, the pointed penial hooks located on laterally-flattened penial papillae, and the vagina having reticulated folds.

Holotype ZMA 3.54.019 (Fig. 6E). Paratypes NHMUK 1954.4.3.3 (1 shell), ZMA 3.54.020 (1 shell), ZMA 3.54.021 (5 shells). Ampang Baru, Ipoh, Perak, Malaysia, 6°29'36.2"N, 100°15'53.2"E, CUMZ 6010 (3 shells; Fig. 6F). Lost World, Tanjung Rambutan, Ipoh, Perak, Malaysia: CUMZ 6007 (9 specimens in ethanol; Figs 2C, 13A, B, 18F–L), 6260 (4 shells).

Shell. Shell depressed-heliciform, white and semi-transparent; whorls 5–5½, spire only slightly convex with distinct suture. Shell surface glossy with transverse ridges and varices present. Whorls regularly coiled; last whorl angular, regularly expanded, ultimate part expanded; umbilicus very wide, deep and showing all preceding whorls. Aperture triangular, sometimes semi-ovate; peristome discontinuous, expanded and little reflected. Apertural dentition of one parietal and one small palatal lamella (Fig. 6E).

Genital organs. Atrium (at) short. Penis (p) long, swollen at middle and with a long and slender penial appendix (pa) about half of penis length. Penial sheath (ps) thin, extending about one-third of penis length; penial sheath retractor muscle (psr) very thin, originating at genital orifice wall and inserting distally on penial sheath (Fig. 13A). Vas deferens (vd) passes through entire length of penial sheath before entering into penis distally (Fig. 13B). Penial retractor muscle (pr) thin and very long, inserting at penis and vas deferens junction.

Internal wall of atrium with smooth surface (Fig. 18F). Penial wall with translucent penial hooks densely scattered, about 18 hooks/200 µm² (Fig. 18G). Hooks located on ovate penial papillae (pp). Penial hooks small (< 0.04 mm in length), tips obtuse and curved towards genital orifice (Fig. 18H–K).

Vagina (v) short, about one-third of penis length. Proximal gametolytic duct (gd) enlarged, stout; distally a long tube extending as far as albumin gland; gametolytic sac (gs) ovate. Proximal free oviduct (fo) enlarged then tapering to smaller tube distally. Oviduct (ov) enlarged and folded; prostate gland inconspicuous and bound to oviduct. Talon (ta) small and club shaped. Hermaphroditic duct (hd) bearing long and thick seminal vesicle (sv) about one and half times longer than the length from talon to branching point of seminal vesicle (Fig. 13A).

Vaginal wall generally smooth (Fig. 18L).

This species is known from the limestone mountains around the type locality in Perak, Malaysia.

Compared with Discartemon platymorphus, this species differs in having a smaller shell, with transverse ridges appearing on the entire shell and a more inflated last whorl. Discartemon leptoglyphus can be distinguished from Discartemon stenostomus by having a depressed-heliciform shell with lower spire, transverse ridges on the entire shell, the last whorl angular, and apertural dentition of one straight parietal lamella. In addition, the penial appendix in Discartemon leptoglyphus is relatively much longer than that shown for Discartemon stenostomus (see Berry 1965).

Holotype ZMA 3.54.022 (fragmented) (Fig. 6G). Paratype ZMA 3.54.023 (2 shells).

The shell is depressed-heliciform with the spire only slightly convex and with a distinct suture. The shell surface has transverse ridges that diminish below the periphery, and varices are present. The following whorls are regularly coiled. Last whorl rounded, regularly expanded; umbilicus widely open and deep. Aperture triangular; peristome discontinuous, thickened, expanded and little reflected; apertural dentition of one parietal and one palatal lamella (Fig. 6H).

Discartemon platymorphus is closely similar to Discartemon plussensis, but that species has a larger shell with a higher spire and lacks a sinulus. Discartemon platymorphus differs from Discartemon epipedis sp. n. by having a lower spire, transverse ridges that diminish below the periphery, a shouldered last whorl, and apertural dentition with four lamellae. Compared with Discartemon stenostomus, Discartemon platymorphus has a lower spire with fine transverse ridges that disappear below the periphery, and a straight parietal lamella and one palatal lamella.

Holotype CUMZ 6249 (Fig. 7A). Measurement: shell height 4.8 mm, shell width 9.3 mm, and with 6½ whorls. Paratypes: CUMZ 4206 (1 shell), 6018 (4 shells), 6019 (23 shells), 6210 (7 specimens in ethanol; Figs 2E, 13C–F, 19A–E, 22D), NHMUK 20130677 (2 shells), and SMF (2 shells) from the type locality.

Wat Khao Pho, Bang Saphan, Prachuap Khirikhan: CUMZ 6012, 6013. Wat Tam Khao Marong, Bang Saphan, Prachuap Khirikhan: CUMZ 4219, 6014, 6211 (5 specimens in ethanol). Bang Saphan Noi, Prachuap Khirikhan: CUMZ 3588. Tam Khao Phlu, Prathiew, Chumphon: CUMZ 3581, 3666, 6214 (3 specimens in ethanol; Figs 2F, 7B). Khao Maeo, Prathiew, Chumphon: CUMZ 3589. Nam Tok Kapo, Tha Sae, Chumphon: CUMZ 3593. Wat Tam Phru-Takien, Tha Sae, Chumphon: CUMZ 6016. Wat Uthai Tam, Chumphon: CUMZ 6212 (6 specimens in ethanol), 6261. Wat Tam Khwan Meuang, Sawi, Chumphon: CUMZ 6015. Suan Somdet, Lang Suan, Chumphon: CUMZ 6017. Tam Khao Krieb, Lang Suan, Chumphon: CUMZ 6213 (3 specimens in ethanol).

Tam Phitsadan, Prathiew, Chumphon, Thailand, 10°43'26.6"N, 99°15'23.6"E.

This new species can be distinguished from Discartemon plussensis, Discartemon leptoglyphus and Discartemon platymorphus in having a nearly smooth shell surface, a shouldered last whorl, and five to seven apertural lamellae. Discartemon afthonodontia sp. n. differs from Discartemon hypocrites by having a nearly smooth shell surface and an aperture with two parietal, two palatal, one basal and two columella lamellae. The genitalia of Discartemon afthonodontia sp. n. differ from those of Discartemon hypocrites in lacking a penial appendix, in having the free oviduct long and slender, and in having the vas deferens passing straight through the penial sheath. They also differ from Discartemon afthonodontia sp. n. in having conical penial papillae, long and slender penial hooks, and in having the penial wall with thick reticulated folds, and the vaginal wall with a smooth surface. Compared with Discartemon epipedis sp. n., Discartemon afthonodontia sp. n. has more apertural lamellae, lacks a penial appendix and has the vas deferens passing straight through the penial sheath. They also differ from Discartemon afthonodontia sp. n. in having a penial wall with thick reticulated folds, and in having a very long and slender free oviduct.

Shell. Shell depressed-heliciform, white and translucent; whorls 6, spire conical to convex with distinct suture. Shell surface glossy, smooth with transverse ridges near the peristome and varices present only on early whorls. Embryonic shell large, about 2½ whorls, with a smooth surface; following whorls regularly coiled. Last whorl shouldered, sometimes angular with strong peripheral keel, regularly expanded, and two shallow and short longitudinal furrows present. Umbilicus widely open and deep. Aperture sub-quadrangular; peristome discontinuous, thickened, expanded and reflected. Aperture dentition with one strong parietal, one palatal, one basal, one large columellar and one small supracolumellar lamella; sometimes upper parietal and upper palatal lamellae present (Fig. 7A).

Radula. Each row consists of 35–39 teeth with formula (17-19)-1-(17-19). Central tooth small with pointed cusp. Lateral and marginal teeth undifferentiated, unicuspid and lanceolate. Latero-marginal teeth gradually reduce in size, with outermost teeth much smaller and shorter than inner teeth (Fig. 22D).

Genital organs. Atrium (at) short. Proximal penis (p) short with very short, stout penial appendix (pa). Distal penis slender (Fig. 13D, E). Penial sheath (ps) thin, extending about one and half times penis length; penial sheath retractor muscle (psr) very thin, originating at genital orifice wall and inserting distally on penial sheath (Fig. 13C). Vas deferens (vd) passes straight through penial sheath before entering into penis distally (Fig. 13D). Penial retractor muscle (pr) thin and very long, inserting at penis and vas deferens junction.

Internal wall of atrium with numerous atrial pores (Fig. 19A). Penial wall with scattered brown penial hooks, about 5 hooks/200 µm² (Fig. 19B). Hooks located on conical penial papillae (pp) which are separated by thickened reticulated folds. Penial hooks small (<0.01 mm in length), expanded at base, tips pointed and curved towards genital orifice (Fig. 19C, D).

Vagina (v) short, about half of penis length. Gametolytic duct (gd) a long tube extending as far as albumin gland; gametolytic sac (gs) ovate. Free oviduct (fo) a very long and slender tube; oviduct (ov) enlarged and folded; prostate gland inconspicuous and bound to oviduct. Talon (ta) small and slender. Hermaphroditic duct (hd) bearing long seminal vesicle (sv) about one and half times longer than the length from talon to branching point of seminal vesicle (Fig. 13C).

Vaginal wall surface generally smooth (Fig. 19E).

The specific epithet “afthonodontia” is derived from the Greek “afthonos” meaning “plenty” and “dontia” meaning “teeth”.

This species is known from several limestone karsts in Chumphon and Prachuap Khirikhan Provinces, southern Thailand. This is a narrow range confined to the Isthmus of Kra area, from 9° to 11° N and 99° to 100° E.

Shell variations are detected across populations. In the Tam Khao Phlu (CUMZ 3581, 3666, 6214) and Khao Maeo (CUMZ 3589) populations, shells have a stronger peripheral keel, a subcircular aperture, and lack the upper parietal lamella (Fig. 7B). The specimens from Wat Tam Khao Marong (CUMZ 4219, 6014, 6211), Wat Tam Khwan Meuang (CUMZ 6015), and Suan Somdet (CUMZ 6017) exhibit a convex spire, and the upper parietal and upper palatal lamellae are sometimes absent. However, these five populations exhibit similar genitalia characters including the penial sculpture. Therefore, we consider them all conspecific.

Holotype CUMZ 6250 (Fig. 7C). Measurement: shell height 4.6 mm, shell width 8.7 mm, and with 6 whorls. Paratypes: CUMZ 6020 (15 shells), 6215 (5 specimens in ethanol; Figs 3A, 14A, B, 19F–I), NHMUK 20130678 (2 shells), and SMF (2 shells) from the type locality.

Gua Matu Madu, Gua Musang, Kelantan, Malaysia, 4°50'13.4"N, 101°56'56.3"E.

Discartemon epipedis sp. n. differs from Discartemon plussensis and Discartemon leptoglyphus in having a higher spire, a nearly smooth shell surface, a semi-ovate aperture, and four apertural lamellae. Compared with Discartemon flavacandida sp. n., Discartemon epipedis sp. n. has a smaller shell, lacks longitudinal furrows, has the last whorl rounded and regularly coiled, and has four apertural lamellae. The genitalia of Discartemon epipedis sp. n. differ from those of Discartemon flavacandida sp. n. in having a very short and swollen penial appendix, a long and enlarged vagina, short free oviduct, low conical penial hooks, penial papillae present, and in lacking vaginal pores. Discartemon epipedis sp. n. differs from Discartemon roebeleni in having a depressed-heliciform shell, a nearly smooth shell surface, and a semi-ovate aperture. The genitalia have a very short and swollen penial appendix, long and enlarged vagina, long and slender free oviduct, dark brown penial hooks located on conical penial papillae, and a vaginal wall with smooth surface.

Shell. Shell depressed-heliciform, white and translucent; whorls 6, spire only slightly convex with distinct suture. Shell surface glossy, nearly smooth with few transverse ridges near peristome; varices present. Embryonic shell large, about 2½ whorls. with a smooth surface; following whorls regularly coiled. Last whorl shouldered or rarely rounded, regularly expanded; umbilicus widely open and deep. Aperture semi-ovate; peristome discontinuous, thickened, expanded and reflected. Apertural dentition with a strong parietal lamella and one palatal, one basal and one columellar lamella (Fig. 7C).

Genital organs. Atrium (at) very short. Proximal penis (p) very short penial appendix (pa) swollen in middle, and distal penis slender. Penial sheath (ps) thin, extending about two-thirds of penis length; penial sheath retractor muscle (psr) very thin, originating at genital orifice wall and inserting distally on penial sheath (Fig. 14A). Vas deferens (vd) passes through about one-seventh of penial sheath length before entering into penis distally (Fig. 14B). Penial retractor muscle (pr) thin and very long, inserting at penis and vas deferens junction.

Internal wall of atrium generally smooth with sparse atrial pores (Fig. 19F); penial wall with scattered dark brown penial hooks, about 2 hooks/200 µm² (Fig. 19G). Hooks located on conical penial papillae (pp) separated by thin reticulated folds. Penial hooks small (<0.03 mm in length), low conical, expanded at base, tips pointed (Fig. 17H).

Vagina (v) long, enlarged, about half of penis length. Gametolytic duct (gd) expanded at base and tapering to long and tube extending as far as albumin gland; gametolytic sac (gs) ovate. Free oviduct (fo) a long and narrow tube; oviduct (ov) enlarged and folded; prostate gland inconspicuous and bound to oviduct. Talon (ta) small, short and club shaped. Hermaphroditic duct (hd) bearing long seminal vesicle (sv) about three times longer than the length from talon to branching point of seminal vesicle (Fig. 14A).

Vaginal wall with smooth surface of strong recticulate vaginal folds (Fig. 17I).

The specific epithet “epipedis” is derived from the Greek “epipedos” meaning “flat” It refers to the flattened- or depressed-heliciform shell.

This species is known only from the type locality.

Apparently rare and only extensive searching yielded living animals.

Holotype CUMZ 6251 (Fig. 7E). Measurement: shell height 5.7 mm, shell width 11.7 mm, and with 6½ whorls. Paratypes: CUMZ 3574 (25 shells), 3576 (9 shells), 3579 (1 shell), 3580 (3 shells), 3675 (7 shells), 3676 (33 shells), 3677 (1 shell), 4214 (26 shells), 6006 (22 shells), 6216 (2 specimens in ethanol; Figs 3B, 14C, D, 19J–N), NHMUK 20130679 (2 shells), and SMF (2 shells) from the type locality.

Tam Phra Khayang, Kra Buri, Ranong, Thailand, 10°19'33.4"N, 98°45'54.7"E.

This new species is distinguished from Discartemon plussensis, Discartemon leptoglyphus, Discartemon platymorphus, Discartemon roebeleni and Discartemon collingei by having a larger shell with a smooth shell surface, a shouldered and slightly axially deflected last whorl, two longitudinal furrows and seven apertural lamellae. Its genitalia are distinctive in having a long but thick penial appendix.

Shell. Shell depressed-heliciform, white and translucent; whorls 6–6½, spire only slightly convex, with distinct suture. Shell surface glossy, smooth with thin growth lines.

Embryonic shell large, about 2½ whorls and with smooth surface; following whorls regularly coiled. Last whorl shouldered, slightly axially deflected, regularly expanded, and two short longitudinal furrows present. Umbilicus widely open and deep. Aperture semi-ovate; peristome discontinuous, expanded and reflected. Apertural dentition with strong parietal and small upper parietal lamellae separated at right angle, one small upper palatal, one palatal, one basal, one columellar, and one small supracolumellar lamella (Fig. 7E).

Genitalia organs: Atrium (at) long and slender. Penis (p) long; proximal penis slender; middle part enlarged with a long but thick penial appendix (pa) about half of penis length; distal penis long and slender (Fig. 14C). Penial sheath (ps) thin, extending about half of penis length; penial sheath retractor muscle (psr) very thin, originating at atrium and inserting distally on penial sheath. Vas deferens (vd) passes through about one-fifth of penial sheath length before entering into penis distally (Fig. 14D). Penial retractor muscle (pr) thin and very long, inserting at penis and vas deferens junction.

Internal wall of atrium generally smooth with numerous atrial pores (Fig. 19J). Penial wall with scattered light brown penial hooks, about 10 hooks/200 µm² (Fig. 19K). Hooks located on penial wall. Penial hooks small (<0.03 mm in length), expanded at base, tips pointed and curved towards genital orifice (Fig. 19L, M).

Vagina (v) very short, about one-fourth of penis length. Gametolytic duct (gd) a long narrow tube extending as far as albumin gland; gametolytic sac (gs) ovate. Free oviduct (fo) very long, slender, proximal with equivalent diameter with vagina, and tapering to smaller tube distally. Oviduct (ov) folded and prostate gland inconspicuous. Talon (ta) small, and very short. Hermaphroditic duct (hd) bearing long seminal vesicle (sv) about one and half times longer than the length from talon to branching point of seminal vesicle (Fig. 14C).

Vaginal wall generally smooth, surface with numerous pores (Fig. 19N).

The specific epithet “flavacandida” is derived from the Latin “flavus” meaning “yellow” and “candidus” meaning “bright or transparent”.

This species seems to be restricted to limestone at the type locality.

Apparently rare and only extensive searching yielded living animals.

Holotype MNHN (Fig. 8A).

Shell globose-heliciform, spire elevated conical with distinct suture; following whorls regularly coiled. Shell surface with transverse ridges; last whorl rounded, regularly expanded; umbilicus unusually narrow. Aperture sub-quadrangular; peristome thickened, expanded and reflected. Apertural dentition with only one parietal lamella.

This species is very superficially similar to Discartemon roebeleni and Discartemon collingei, but has a larger shell with higher spire, unusually narrow umbilicus, and larger aperture with only a parietal lamella. In addition, the three species are allopatric, with Discartemon lemyrei occurring in Kampot and Panompen of Cambodia, while Discartemon roebeleni and Discartemon collingei occur in southern Thailand and peninsular Malaysia.

Lectotype of Streptaxis roebeleni SMF 108526 (Fig. 8B), and paralectotypes SMF 108527 (5 shells), 108528 (2 shells), 108529 (1 shell), 108530 (1 shell).

Holotype of forma major SMF 108531 (Fig. 8C), and paratype 108532 (2 shells). Holotype of forma minor SMF 108533 (Fig. 8D).

Topotypes from Samui, Thailand: NMW 1955.158.25255 (1 shell), and Nam Tok Hin Lad, Samui, Suratthani, Thailand, 9°31'15.3"N, 99°57'20.1"E: CUMZ 3655 (Fig. 8E), 4217, 6217 (52 specimens in ethanol; Figs 3C, 15A–C, 20A–E, 22E).

Topotypes from Samui, Thailand: NMW 1955.158.25255 (1 shell), and Nam Tok Hin Lad, Samui, Suratthani, Thailand, 9°31'15.3"N, 99°57'20.1"E: CUMZ 3655 (Fig. 8E), 4217, 6217 (52 specimens in ethanol; Figs 3C, 15A–C, 20A–E, 22E). Samui Island, Gulf of Siam [Thailand]: NHMW 36538 (1 shell), NHMW Rusnov R284 (1 shell), RMNH Fulton Coll. Reg. 177 (2 shells), ZMB 43127 (2 shells). Kow Tao Is. [=Ko Tao], Thailand: NMW 1955.158.25254 (7 shells). Ko Tao, Suratthani: CUMZ 3577. Ko Wuatalub, Ang Thong National Park, Suratthani: CUMZ 6022, 6218 (1 specimen in ethanol). Ko Mae Ko, Ang Thong National Park, Suratthani: CUMZ 6219 (1 specimen in ethanol). Ban Ta Khun, Suratthani: CUMZ 3590. Ratchaprapha reservior, Ban Ta Khun, Suratthani: CUMZ 6220 (4 specimens in ethanol). Khlong Saeng Wildlife Sanctuary, Ban Ta Khun, Suratthani: CUMZ 3652. Wat Khao Khok, Wiang Sa, Suratthani: CUMZ 3658. Wat Na San, Ban Na San, Suratthani: CUMZ 3578. Km 3, Khiri Rat Nikhom, Suratthani: CUMZ 6221 (1 specimen in ethanol). Wat Khao Phanom Wang, Suratthani: CUMZ 6222 (1 specimen in ethanol). Tam Hong, Khao Nan National Park, Nakhon Si Thammarat: CUMZ 4221. Tam Luang, Khao Nan National Park, Nakhon Si Thammarat: CUMZ 4231. Tam Phannara, Nakhon Si Thammarat: CUMZ 3667. Tam Khun Klung, Nopphitam, Nakhon Si Thammarat: CUMZ 6021. Khao Phrathong, Cha-uat, Nakhon Si Thammarat: CUMZ 3599. Wat Suwankhuha, Takua Thung, Phangnga: CUMZ 3661 (Fig. 8F), 6223 (14 specimens in ethanol). Sra Morakot, Krabi: CUMZ 6023, 6226 (7 specimens in ethanol). Ao Phra Nang, Krabi: CUMZ 3651. Khao Huai Hang, Huai Yot, Trang: CUMZ 3656. Tam Lay-Kao Krop, Huai Yot, Trang: CUMZ 3600. Botanic Garden, Trang: CUMZ 3663. Khao Pi-na, Na Yong, Trang: CUMZ 6024. Tam Sumno, Trang: CUMZ 6225 (1 specimen in ethanol). Khao Pu-Khao Ya National Park, Si Banphot, Phatthalung: CUMZ 3575, 3596. Tam Wang Thong, Phatthalung: CUMZ 3662, 6027, 6224 (6 specimens in ethanol). Wat Khaotupson, Phatthalung: CUMZ 3678. Khao Ok Thalu, Phatthalung: CUMZ 3595. Khao Chaison, Phatthalung: CUMZ 6028. Tam Tanan, Satun: CUMZ 6025. Tam Khantiphon, Satun: CUMZ 6026, 6227 (1 specimen in ethanol). Ko Buloan Pai, La Ngu, Satun: CUMZ 3591. Ko Tarutao, Satun: CUMZ 6228 (7 specimens in ethanol), 6256 (Fig. 8G). Ko Klang, Tarutao, Satun: CUMZ 6229 (9 specimens in ethanol). Khao Nui, Rattaphum, Songkhla: CUMZ 3598.

Shell. Shell globose-heliciform, white and translucent; whorls 6–6½, spire conical with distinct suture. Shell surface glossy, with transverse ridges that diminish below periphery; varices present. Embryonic shell large, about 2½ whorls, with a smooth surface; following whorls regularly coiled. Last whorl rounded, regularly expanded; umbilicus widely open and deep. Aperture sub-quadrangular; peristome discontinuous, thickened, expanded and reflected. Apertural dentition with one strong parietal, one palatal, one basal and one columellar lamella (Fig. 8B). Sometimes basal lamella absent (Fig. 8F), or upper palatal and supracolumellar lamellae present (Fig. 8G).

Radula. Each row consists of 21–33 teeth with formula (10-16)-1-(10-16). Central tooth very small and triangular with a pointed cusp. Lateral and marginal teeth undifferentiated, unicuspid and lanceolate. Latero-marginal teeth gradually reduce in size, with outermost teeth much smaller and shorter than inner teeth (Fig. 22E).

Genitalia organs. Atrium (at) long. Penis short and slender. Penial sheath (ps) extending entire penis length; penial sheath retractor muscle (psr) very thin, originating at genital orifice wall and inserting distally on penial sheath (Fig. 15A). Vas deferens (vd) passes through about one-seventh of penial sheath length before entering into penis distally (Fig. 15B). Penial retractor muscle (pr) thin and very long, inserting at penis and vas deferens junction.

Internal wall of atrium generally corrugated (Fig. 20A). Penial wall with scattered, transparent penial hooks, about 8 hooks/200 µm². Hooks located on very short penial papilla (pp). Penial hooks small (<0.03 mm in length), short, expanded at base, tips pointed and curved towards genital orifice (Fig. 20C, D).

Vagina (v) short, about half of penis length. Gametolytic duct (gd) a long and slender tube extending as far as albumin gland; gametolytic sac (gs) ovate. Free oviduct (fo) short, about same length as vagina; oviduct (ov) folded; prostate gland inconspicuous and bound to oviduct. Talon (ta) small, very short. Hermaphroditic duct (hd) bearing extremely long seminal vesicle (sv) (Fig. 15C).

Vaginal wall generally with longitudinal vaginal folds (Fig. 20E).

This species is found in limestone habitats and is common in southern Thailand. The geographic distribution records are in eight Provinces, ranging from 10°N to 6°N: Suratthani, Nakhon Si Thammarat, Krabi, Phangnga, Phatthalung, Trang, Satun, and Songkhla.

Discartemon roebeleni can be distinguished from Discartemon collingei by its rounded, regularly coiled last whorl and wider umbilicus. It differs from Discartemon stenostomus in the higher spire with transverse ridges that diminish below the periphery, a sub-quadrangular aperture, and in having a basal lamella.

Syntype NHMUK 1937.7.9.20 (1 shell; Fig. 8H). Kelantan, Malaysia: NMW 1955.158.25250 (2 shells). Kelantan, Malaysia: NHMW 40716 (1 shell).

Shell globose-heliciform, translucent, with a conical spire with a distinct suture. Shell surface with transverse ridges that diminish below periphery; varices present. Later whorls slightly axially deflected. Last whorl shouldered and regularly expanded; umbilicus wide and deep. Aperture sub-quadrangular; peristome thick, expanded and reflected. Apertural dentition with one parietal, one palatal, one basal and one columellar lamella (Fig. 8H).

Discartemon collingei is similar to Discartemon lemyrei, but the latter species has a larger shell with a higher spire, a shell surface with transverse ridges, the last whorl rounded and more inflated, a narrower umbilicus, and only one parietal lamella.

Holotype ZMA 3.54.024 (Fig. 9A). Paratypes ZMA 3.54.025 (8 shells), and NHMUK 1957-4-3.1-2 (2 shells). Kaki Bukit, Perlis: NMW 1955.158.25256 (10 shells).

Shell globose-heliciform, semi-transparent, spire only slightly convex and with a distinct suture. Shell surface glossy with thin transverse ridges at suture; following whorls regularly coiled. Last whorl rounded and regularly expanded; umbilicus widely open and deep. Aperture triangular; peristome thickened, expanded and reflected. Apertural dentition with one sinuous parietal, one palatal, one columellar and one supracolumellar lamella (Fig. 9A).

The genital anatomy was described by Berry (1965). Atrium and penis short with blunt penial appendix, penial sheath extending almost entire penis length, vas deferens passing through about one-fifth of penial sheath length. Internal wall of penis corrugated with cornified ridges, penial hooks absent, but with a large hollow “stylet” presumably protrudable from the tip of the everted penis. Vagina very short, proximal gametolytic duct enlarged, distally a long slender tube. Free oviduct short; talon small, club shaped; seminal vesicle about the same length from talon to branching point of seminal vesicle. Internal wall of vaginal elaborated with parallel vaginal folds.

This species resembles Discartemon lemyrei and Discartemon collingei, but differs in having thin transverse ridges near the suture, a triangular aperture, and apertural dentition with a sinuous parietal lamella, one palatal, one columellar and one supracolumellar lamella. In addition, Discartemon lemyrei has a larger shell, narrower umbilicus, and only one parietal lamella, while Discartemon collingei has a shouldered and slightly axially reflected last whorl, and apertural dentition with a straight parietal, one palatal, one basal and one columellar lamellae. A penial stylet has not yet been found in any other Discartemon species.

Holotype ZMA 3.59.052 (Fig. 9C). Paratypes: ZMA 3.59.053 (1 shell), ZMA 3.59.054 (4 shells), and ZMA 3.59.057 (9 shells).

Shell globose-heliciform, semi-transparent, with a conical spire and distinct suture. Shell surface glossy with fine transverse ridges; following whorls regularly coiled. Last whorl rounded and regularly expanded; umbilicus widely open and deep. Aperture triangular, with sinulus; peristome thickened, expanded and reflected. Apertural dentition of only one parietal lamella (Fig. 9C).

Discartemon sangkarensis differs from Discartemon roebeleni in having a more inflated last whorl, a triangular aperture, in having a sinulus, and the apertural dentition of only one parietal lamella. Compared with Discartemon lemyrei and Discartemon vandermeermohri, Discartemon sangkarensis differs in having a sinulus. Also, Discartemon lemyrei has a relatively larger shell and narrow umbilicus, while Discartemon vandermeermohri has a small basal lamella. Discartemon sangkarensis differs from Discartemon collingei in having a higher spire, the last whorl rounded, more inflated and regularly coiled, and in having a sinulus, a triangular aperture, and only one parietal lamella. Also, Discartemon collingei is slightly axially deflected.

Holotype ZMA 3.59.055 (Fig. 9E). Paratypes ZMA 3.59.056 (2 shells) and RMNH Brandhorst Reg. 387 (1 shell).

Shell thickened, globose-heliciform, with a conical spire and distinct suture. Shell surface with strong transverse ridges; varices present; following whorls regularly coiled. Last whorl rounded and regularly expanded; umbilicus widely open and deep. Aperture triangular; peristome thickened, expanded and reflected. Apertural dentition of one parietal and one columellar lamella (Fig. 9E).

This species differs from Discartemon lemyrei in its smaller shell, widely open umbilicus, triangular aperture, and in having two apertural lamellae. Discartemon vandermeermohri is readily distinguished from Discartemon roebeleni and Discartemon collingei in its having a triangular aperture, and in lacking a basal lamella. Also, Discartemon collingei is slightly axially deflected.

Holotype CUMZ 6252 (Fig. 9G). Measurement: shell height 6.3 mm, shell width 9.2 mm, and with 6 whorls. Paratypes: CUMZ 4220 (27 shells), 6230 (15 specimens in ethanol; Figs 3D, 15D–F, 20F–J, 22F), NHMUK 20130680 (2 shells), and SMF (2 shells) from the type locality.

Ko Tan, Samui, Suratthani, Thailand, 9°22'18.9"N, 99°56'53.7"E.

Nam Tok Tone Nga Chang, Had Yai, Songkhla: CUMZ 6231 (4 specimens in ethanol). Ban Chang Lang, Si-kao, Trang: CUMZ 6232 (2 specimens in ethanol).

Conchologically this new species superficially resembles Discartemon roebeleni and Discartemon megalostraka sp. n. It differs from Discartemon roebeleni in having a higher spire, a very long penis, a penial sheath extending fourth-fifths of the penis length, a smooth atrium wall with atrial pores, and a short seminal vesicle. It differs from Discartemon megalostraka sp. n. in having a smaller shell and apertural dentition of four lamellae, and a shorter free oviduct, vas deferens and seminal vesicle. Discartemon kotanensis sp. n. differs from Discartemon stenostomus and Discartemon collingei in having a higher spire, transverse ridges reaching the periphery, the last whorl rounded and regularly coiled, and apertural dentition of one straight parietal, one basal and one columellar lamella. Also, Discartemon collingei is slightly axially deflected.

Shell. Shell globose-heliciform, white and translucent; whorls 6–6½, spire elevated conical, with distinct suture. Shell surface glossy, with transverse ridges that diminish below periphery; varices present. Embryonic shell large, about 2½ whorls, with a smooth surface; following whorls regularly coiled. Last whorl rounded and regularly expanded; umbilicus widely open and deep. Aperture sub-quadrangular; peristome discontinuous, thickened, expanded and reflected. Apertural dentition of one strong parietal, one palatal, one basal and one columellar lamella (Fig. 9G).

Radula. Each row consists of 27–31 teeth with formula (13-15)-1-(13-15). The central tooth is very small and triangular with a pointed cusp. Lateral and marginal teeth are undifferentiated, unicuspid and lanceolate. Latero-marginal teeth gradually reduce in size, with outermost teeth much smaller and shorter than inner teeth (Fig. 22F).

Genital organs. Atrium (at) short. Penis (p) very long and slender. Penial sheath (ps) thin and extending about fourth-fifths of penis length, and penial sheath retractor muscle (psr) very thin, originating at genital orifice wall and inserting distally on penial sheath (Fig. 15D). Vas deferens (vd) passes a very short distance through penial sheath before entering into penis distally (Fig. 15E). Penial retractor muscle (pr) thin and very long, inserting at penis and vas deferens junction.

Internal wall of atrium generally smooth with atrial pores (Fig. 20F). Penial wall with dense and transparent penial hooks, about 20 hooks/200 µm² (Fig. 20H). Hooks located on short penial papillae (pp). Penial hooks small (<0.03 mm in length), expanded at base, tips pointed and curved towards genital orifice (Fig. 20I).

Vagina (v) short, about one-fifth of penis length. Gametolytic duct (gd) a long tube extending as far as albumin gland; gametolytic sac (gs) ovate. Free oviduct (fo) short, of about same length as vagina. Oviduct (ov) slender and folded; prostate gland inconspicuous. Talon (ta) small, very short and club shaped. Hermaphroditic duct (hd) bearing long seminal vesicle (sv) about twice as long as the length from talon to branching point of seminal vesicle (Fig. 15F).

Vaginal wall generally with longitudinal vaginal folds (Fig. 20J).

The specific epithet is derived from the type locality of this new species, the Ko Tan, Ko Samui, Suratthani Province.

This species is known from the type locality and few limestone outcrops on the southern mainland.

Shells of this species from Samui, Suratthani were originally thought to belong to Discartemon roebeleni. After the genital system of Discartemon kotanensis sp. n. was examined and critically investigated, it was considered distinct enough to be a separate species.

Holotype CUMZ 6253 (Fig. 10A). Measurement: shell height 8.0 mm, shell width 12.0 mm, and with 7 whorls. Paratypes: CUMZ 3657 (5 shells), 6031 (9 shells), 6233 (3 specimens in ethanol), NHMUK 20130681 (2 shells), and SMF (2 shells) from the type locality.

Nam Tok Tao Thong, Tub Pud, Phangnga, Thailand, 8°29'0.8"N, 98°35'4.8"E.

Wat Tam Seua, Krabi: CUMZ 6029. Ban Chong, Krabi: CUMZ 6030. Wat Sathit Khirirom, Khirirat Nikhom, Suratthani: CUMZ 6234 (1 specimen in ethanol). Tam Wang Badan, Suratthani: CUMZ 6235 (2 specimens in ethanol).

This species differs from Discartemon lemyrei in its widely open umbilicus and apertural dentition of six lamellae. It differs from Discartemon roebeleni in having a much larger shell, higher spire, upper palatal and supracolumellar lamellae, a very long penis, penial sheath and free oviduct, shorter seminal vesicle, and in having atrial pores. Discartemon megalostraka sp. n. can be distinguished from Discartemon stenostomus in its larger shell, higher spire, transverse ridges, sub-quadrangular aperture, its straight parietal lamella, and in having upper palatal and basal lamellae.

Shell. Shell globose-heliciform, white and translucent; whorls 7–7½, spire elevated conical, with a distinct suture. Shell surface glossy with fine transverse ridges that diminish below periphery; varices present. Embryonic shell large, about 2½ whorls, with a smooth surface; following whorls regularly coiled. Last whorl rounded and regularly expanded; umbilicus widely open and deep. Aperture sub-quadrangular; peristome discontinuous, thickened, expanded and reflected. Apertural dentition of one strong parietal, one small upper palatal, one palatal, one basal, one columellar and one small supracolumellar lamella (Fig. 10A).

Radula. Each row consists of 19–21 teeth with formula (9-10)-1-(9-10). The central tooth is small and triangular with a pointed cusp. Lateral and marginal teeth are undifferentiated and large, unicuspid and lanceolate. Latero-marginal teeth gradually reduce in size, with outermost teeth much smaller and shorter than inner teeth (Fig. 22G).

Genital organs. Atrium (at) very short. Penis (p) very long and slender. Penial sheath (ps) thin, extending about third-fourths of penis length. Penial sheath retractor muscle (psr) very thin, originating at genital orifice wall and inserting distally on penial sheath (Fig. 16A). Vas deferens (vd) passes a very short distance through penial sheath before entering into penis distally (Fig. 16B). Penial retractor muscle (pr) thin and long, inserting at penis and vas deferens junction.

Internal wall of atrium with large atrial pores (Fig. 21A). Penial wall with scattered and transparent penial hooks, about 9 hooks/200 µm² (Fig. 21B). Hooks located on penial wall. Penial hooks small (<0.02 mm in length), expanded at base, tips pointed and curved towards genital orifice (Fig. 21C–E).

Vagina (v) very short. Gametolytic duct (gd) a long and narrow tube extending as far as albumin gland; gametolytic sac (gs) ovate. Free oviduct (fo) extremely long, proximal part a straight cylindrical tube, distal part corrugated. Oviduct (ov) enlarged and folded; prostate gland inconspicuous. Talon (ta) small, very short and club shaped. Hermaphroditic duct (hd) bearing a long seminal vesicle (sv) about four times as long as the length from talon to branching point of seminal vesicle (Fig. 16C).

Vaginal wall with longitudinal vaginal folds (Fig. 21F).

The specific epithet “megalostraka” is derived from the Greek “megalos” meaning “big” and “ostrako” meaning “shell”.

This species is known from several limestone hills in southern Thailand, particularly in the western part of the southern mainland. The animals can be found at altitudes up to 20 meters amsl.

The genital system discriminates this new species from large individuals of Discartemon roebeleni, which is distributed throughout southern Thailand.

Holotype CUMZ 6254 (Fig. 10C). Measurement: shell height 4.6 mm, shell width 7.3 mm, and with 6 whorls. Paratypes: CUMZ 6032 (2 shells), 6236 (6 specimens in ethanol), and NHMUK 20130682 (2 shells) from the type locality.

Gunung Kilian, Perlis, Malaysia, 6°34'8.0"N, 100°11'44.4"E.

This new species is superficially similar to Discartemon roebeleni and Discartemon kotanensis sp. n., but the distinguishing characters are the smaller shell, lower spire, angular last whorl, very long penis and free oviduct, short seminal vesicle, and penial hooks with elongated bases. Discartemon triancus sp. n. can be distinguished from Discartemon megalostraka sp. n. by having a smaller shell, lower spire, four apertural lamellae, a longer penis, short free oviduct, and slender penial hooks with elongated bases. Discartemon triancus sp. n. differs from Discartemon conicus sp. n. in having a lower spire with shallow suture, transverse ridges, in lacking a sinulus, and in having four apertural lamellae.

Shell. Shell globose-heliciform, white and translucent; whorls 5½–6, spire only slightly convex, with distinct suture. Shell surface glossy with transverse ridges that diminish below the periphery; varices present. Embryonic shell large, about 2½ whorls, with a smooth surface; following whorls regularly coiled. Last whorl angular, regularly expanded; umbilicus widely open and deep. Aperture subcircular; peristome discontinuous, thin and expanded. Apertural dentition of one parietal, one palatal, one small basal and one columellar lamella (Fig. 10C).

Radula. Each row consists of 27–43 teeth with formula (13-21)-1-(13-21). The central tooth is very small with pointed cusp. Lateral and marginal teeth are undifferentiated, unicuspid and lanceolate. Latero-marginal teeth gradually reduce in size, with outermost teeth much smaller and shorter than inner teeth (Fig. 22H).

Genital organs. Atrium (at) very short. Penis (p) extremely thin, long; becoming enlarged distally. Penial sheath (ps) thin, extending about half of penis length. Penial sheath retractor muscle very thin (psr), originating at atrium and inserting distally on penial sheath (Fig. 16D). Vas deferens (vd) passes a very short distance through penial sheath before entering into penis distally (Fig. 16E). Penial retractor muscle (pr) thin and very long, inserting at penis and vas deferens junction.

Internal wall of atrium generally smooth with pores (Fig. 21G). Penial wall with scattered and transparent penial hooks, about 11 hooks/200 µm² (Fig. 21H). Hooks located on penial wall. Penial hooks small (<0.04 mm in length), short, with strongly elongated bases, tips pointed, and curved towards genital orifice (Fig. 21I–K).

Vagina (v) short. Gametolytic duct (gd) a long and slender tube extending as far as albumin gland; gametolytic sac (gs) ovate. Free oviduct (fo) with almost same diameter as vagina and about twice as long as vagina. Oviduct (ov) enlarged and folded; prostate gland inconspicuous. Talon (ta) small, short and slender. Hermaphroditic duct (hd) bearing a short seminal vesicle (sv) nearly equal to the length from talon to branching point of seminal vesicle (Fig. 16F).

Vaginal wall with longitudinal vaginal folds (Fig. 21L).

The specific epithet “triancus” is derived from the Latin “triangulum” meaning “triangle” and “uncus” meaning “hook”.

Known only from the type locality.

Material from Gunung Kilian, Perlis, Malaysia was firstly identified as Discartemon roebeleni (Möllendorff, 1894) by Benthem Jutting (1954), without any anatomical comparison. However, clear anatomical differences between this new species and Discartemon roebeleni, so it is considered a new species.

Holotype CUMZ 6255 (Fig. 10E). Measurement: shell height 4.5 mm, shell width 7.2 mm, and with 6 whorls. Paratypes: CUMZ 6033 (2 shells) from the type locality.

Gau Cerita, Langawi, Malaysia, 6°27'21.8"N, 99°49'29.8"E.

This species differs from Discartemon roebeleni, Discartemon sangkarensis, Discartemon vandermeermohri and Discartemon kotanensis sp. n., in having a smaller shell, higher spire, a nearly smooth shell surface, an angular last whorl, a sub-quadrangular aperture with a sinulus, and in having only one parietal lamella.

Shell. Shell globose-heliciform, white and translucent; whorls 6, spire elevated conical, with distinct suture. Shell surface glossy, smooth with thin transverse ridges near aperture; varices present. Embryonic shell large, about 2½ whorls, with a smooth surface; following whorls regularly coiled. Last whorl angular, inflated and regularly expanded. Umbilicus open and deep. Aperture sub-quadrangular with sinulus; peristome discontinuous, expanded and reflected. Apertural dentition of only one parietal lamella (Fig. 10E).

Known only from the type locality among limestone karsts up to 100 meters amsl, surrounded by mangrove forests on the northeast Langkawi Island coastline.

The new species is apparently rare and extensive searching yielded only three examples.