Systematic review of the dextral Hemiplecta Albers, 1850 (Eupulmonata, Ariophantidae) from Thailand with description of a new species and list of all the Indochinese species

Abstract The genus Hemiplecta is a group of large-sized land snails which have long been used as a food resource by Indochinese people. There are five dextral and four sinistral species currently recognized from Thailand. The dextral group is comprised of two previously recorded species (H. humphreysiana and H. distincta), two newly recorded species (H. funerea and H. esculenta), and one new species (H. nemorosasp. nov.) from northern Thailand is being proposed. We reassessed the diagnostic characters of the genitalia, mantle edge, and radula. Specimens were classified into the genus Hemiplecta on the basis of the penial verge and shell lobe, and on the characters of a bulbous gametolytic sac without a gametolytic duct. A complete species list, together with photographs of the name-bearing types or authenticated specimens and the taxonomic status of Hemiplecta s.l. that are known from Indochina including Peninsular Malaysia and Myanmar, is provided for the first time. In total, this species list contains 39 available nominal species names described from this area. Type or authentic specimens can be located for 37 nominal species names, of which 25 are illustrated herein and the other 12 were recently illustrated. However, two available species-level names could not be traced to any type specimens. In addition, lectotypes of H. funerea and H. pluto are designated herein to stabilize the names.


Introduction
As currently understood, the diverse ariophantid snail genus Hemiplecta Albers, 1850 consists of around 50 dextral species as well as five sinistral species (MolluscaBase 2020; Sutcharit et al. 2021). They are distributed from Indochina through the Sunda Islands to New Guinea (Zilch 1959;Schileyko 2002), with vague records from the Maldives and Kerala, India (Schileyko 2002;Ramakrishna et al. 2010). The genus contains large ariophantid species (shell width up to 70 mm), and at least two species, H. distincta (Pfeiffer, 1850) and H. esculenta Maassen, 2006, have been known as food for local people in northeastern Thailand, Laos, Cambodia, and Vietnam. It is also an intermediate host of the rat lungworm (Panha 1987b(Panha , 1988b(Panha , 1994Maassen 2006;Tesana et al. 2009;Mienis et al. 2010).
Taxonomically, Hemiplecta was first established as a distinct section [? subgenus] of Nanina Gray, 1834 with a brief definition to contain large helicoid species mainly from the Philippines and the Malay Archipelago (Albers 1850). It was then reassessed with an extended generic description and the addition of several species from India and Southeast Asia (Albers 1860; Godwin-Austen 1888b). Later, it was formally treated as a subgenus of Nanina (Adams and Adams 1858;Nevill 1878), but this was not widely accepted. The generic demarcation of Hemiplecta was systematically revised based on anatomical characters of its type species by Godwin-Austen (1897). In addition to their shell morphology, species attributed to this genus have a well-developed dart apparatus and a bulbous gametolytic sac without a duct (Godwin-Austen 1897). These characters were accepted as being more reliable than the shell morphology, and were followed until recently (Blanford and Godwin-Austen 1908;Thiele 1931;Zilch 1959;Schileyko 2002). The recent phylogeny of some Indochinese Hemiplecta (including the type species) confirm that they are monophyletic and are comprised not only of dextral species but also several sinistral species that were previously included in the Dyakia Godwin-Austen, 1891 (see Sutcharit et al. 2021). In addition, the modern systematic revision of various helicarionoid groups has illustrated the taxonomic importance of reproductive characters for distinguishing taxa at both the generic and specific levels (i.e. Hyman and Ponder 2010;Köhler 2018, 2019a, b). Until now, the taxonomic treatment of many Hemiplecta species has been confusing and remained contentious due to the paucity of crucial reproductive characters.
In the present study, we aimed to establish a stable and objective taxonomy by incorporating data from the reproductive organs, pallial system and radula morphology. All recognized and undescribed dextral Hemiplecta species occurring in Thailand were critically examined, and their morphological variation and distribution ranges are presented. Previously, most of the Hemiplecta species have been described based solely on their shells. However, where anatomical data for additional Hemiplecta species was available in the literature, this was summarized and compared with the results of the present study. Furthermore, all the nominal taxa currently attributed to the genus Hemiplecta s.l. that have the type locality in Indochina, Peninsular Malaysia and Myanmar are alphabetically listed. In addition, the primary type specimens or authentic specimens (when possible) are figured for further comparisons and precise identification.

Materials and methods
Snails were sampled throughout Thailand. Living snails were euthanized by the two-step method (AVMA 2020), then transferred to 70% (v/v) ethanol for fixation, preservation, and subsequent anatomical study. Genital systems of up to five specimens of each species were examined. Radulae were extracted, and examined under scanning electron microscopy (SEM; JEOL, JSM-5410 LV). Radula shape and teeth formula were analyzed.

CUMZ
Chulalongkorn Remarks. Due to the high degree of similarity in shell morphology, the specific and generic classification within Ariophantidae is usually problematic. There are at least three nominal genera that are often confused, Nanina Gray, 1834, Ariophanta Des Moulins, 1829and Cryptozona Mörch, 1872. The genitalia have proved to be the distinguishing characters for specific or generic recognition among the Ariophantidae (Laidlaw 1932a;Solem 1966;Sutcharit and Panha 2008). However, only a few species of each genus have been anatomically examined. Based on this limited anatomical information, the unique characters taken from the type species are summarized in Table 1. The genus Hemiplecta can be differentiated from these three genera by lacking a gametolytic duct, while the others possess a short to long gametolytic duct. Hemiplecta also differs from Ariophanta and Cryptozona by having unicuspid central teeth, very short or absent flagellum, and a mantle edge with shell lobes; while the latter two genera have tricuspid central teeth, long flagellum, and a mantle edge without shell lobes. In addition, Hemiplecta can be distinguished from Nanina by its short flagellum (Table 1).
Shell. Shell large (height up to 40 mm, width up to 55 mm), dextral and conic to depressed conic ( Fig. 2A, B). Whorls 6 to 8, slightly convex; suture wide and shallow. Shell yellowish to brownish, usually with narrow dark brown band on periphery. Upper shell surface darker than lower surface. Apex obtuse; embryonic shell large and smooth; following whorls with thin growth lines. Last whorl rounded to slightly angulate; aperture ovate; lip simple but slightly thickened in adult snail. Columella slightly dilated; parietal callus thin and translucent. Umbilicus open and deep.
Genitalia. Atrium (at) very short (Fig. 3A). Penis (p) long, slender, cylindrical, and encircled by thin penial sheath (psh) extending about one-third of penis length. Epiphallic caecum (ec) short, straight; penial retractor muscle (pr) thin and attached to the tip. Epiphallus (e) short and about one-third of penis length. Flagellum (fl) short, stout, and with thin muscle bands connected to penial sheath. Vas deferens (vd) small tube. Internal wall of penis with sculpture encircling penial verge (Fig. 3B). Penial sculpture (ps) consists of scattering of small papillary knobs arranged randomly on penial wall. Penial verge (pv) long conic with smooth surface.
External features. Mantle edge with large dorsal lobes. Right dorsal lobe (rdl) to right of anus (an; on the left in figure), large, and thick. Left dorsal lobe to left of anus (on the right in figure), composed of thin crescentic anterior left dorsal lobe (aldl), and thin elongated posterior left dorsal lobe (pldl). Right shell lobe (rsl) and left shell lobe (lsl) have short finger-shaped extensions located on mantle edge near tip of urinary groove and around junction of anterior and posterior left dorsal lobes, respectively, (Fig. 3C, D).
Pulmonary cavity typically sigmurethran, heart (h; auricle and ventricle) located left of kidney (k; on the right in figure). Pulmonary cavity approximately four times longer than wide. Pulmonary vein (puv) and venation on lung cavity well developed and distinct. Kidney (k) elongate, slender, and approximately one-third length of pulmonary cavity. Ureter (ur) sigmoid, closed tube arising from tip of kidney, extending along right side of kidney, and curved adjacent to rectum (r). Anus (an) adjacent to mantle edge (Fig. 3C).
Living snails possess long greyish-brown tentacles (Fig. 1A). Skin reticulated brownish with blackish reticulations around head. Foot sole relatively elongate, broad and unipartite. Sole of foot plain brownish; side of body brownish; upper part of tail dark greyish. Tail long, curved mid-dorsally, tall dome-shaped in cross section. Caudal horn not overhanging; caudal foss a long vertical slit arranged on tail above sole margin. Pedal groove typical aulacopod and well defined (Fig. 3E).
Genitalia. The external genital organs were described in Godwin-Austen (1900, 1919. Gametolytic sac (gs) bulbous with undifferentiated duct. Internal wall of penis exhibits closely packed papilla knobs that abruptly cease near atrium; penial verge absent. Internal sculpture of vagina with thin and smooth longitudinal vaginal pilasters (vp). Internal surface of dart apparatus smooth; dart papilla (dp) conic, and with a smooth surface (Fig. 4A, B).
External features. Living snails have a similar soft body morphology and pulmonary cavity to that of H. humphreysiana. The distinct characters are pale brown to brownish body (Fig. 1B, C). Sole of foot brownish; caudal horn not overhanging; caudal foss a long vertical slit arranged on tail above sole margin. Pedal groove typical aulacopod and well defined (Fig. 4E). Mantle edge narrow with large dorsal lobes. Right dorsal lobe (rdl) to right of pneumostome, large and thick; left dorsal lobe to left of pneumostome, composed of anterior left dorsal lobe (aldl) and posterior left dorsal lobe (pldl); shell lobe absent (Fig. 4C, D).
Distribution. Ranges from Cambodia to Laos, Thailand and southern Vietnam (Smith 1896;Laidlaw 1932a;Panha 1988aPanha , 1994Schileyko 2011;Inkhavilay et al. 2019). In Thailand, H. distincta is fairly abundant and occurs throughout the country, except for southern Thailand (Panha 1988b(Panha , 1994. The southern limit of the species appears to be near the Isthmus of Kra (10°N). We have a single and old shell from Pangnga Province, southern Thailand that needs to be confirmed.
Remarks. The type specimens of Helix neptunus Pfeiffer, 1861 and Hemiplecta zimmayensis Godwin-Austen, 1888 exhibit a shell morphology and color patterns identical to that of H. distincta. The absence of a whitish peripheral band in Helix neptunus Pfeiffer, 1861 and the strong growth lines of Hemiplecta zimmayensis are the only observed differences from H. distincta. Therefore, we recognize these two nominal species as junior subjective synonyms of H. distincta. Hemiplecta distincta has long been considered a food item for local people in northeastern Thailand (Panha 1987b), as well as in Cambodia and Laos (personal observation). It has also been found to be an intermediate host of the rat lungworm, a human pathogen (Panha 1987b(Panha , 1988b. The life cycle and breeding biology of this species have been extensively studied (Panha 1987a(Panha , b, 1988a(Panha , b, 1994. Figures 1D, E, 5A Shell. Shell large (height up to 35 mm, width up to 55 mm), depressed conic, dextral, with 6-7 whorls; spire slightly elevated with wide and shallow suture. Shell almost black to dark brown with thin yellowish peripheral band. Apex obtuse; embryonic shell large with smooth surface; subsequent whorls with thin growth lines and thin radial wrinkles. Last whorl keeled; aperture large and ovate; lip simple, yellowish to dark yellow, and slightly thickened in adult snail. Columella slightly dilated; parietal callus thin and transparent. Umbilicus wide and deep (Fig. 5A, B).

Hemiplecta funerea (Smith, 1896)
Genitalia. Both male and female genital characters similar to that of H. humphreysiana. Gametolytic sac (gs) elongate with undifferentiated duct. The unique characters are a coiled epiphallic caecum (ec) and curved flagellum (fl), which are not found in the other species (Fig. 6A). Internally, penial sculpture (ps) consists of scattered papillary knobs lining penial wall; penial verge absent (Fig. 6B). Internal wall of vagina and internal structure of dart apparatus are similar to that in H. humphreysiana (Fig. 6C, D).
External features. Living snails with long, black eye tentacles (Fig. 1D, E). Skin reticulated, pale brownish to brownish with dark reticulation across the entire head and foot above the lateral margin. Foot sole, caudal foss (Fig. 6G), caudal horn, and pedal groove similar to those in H. humphreysiana. Mantle edge, dorsal lobe, and shell lobe similar to those in H. humphreysiana, but only long and finger-shaped right shell lobe (rsl) present (Fig. 6E, F).  Distribution. Previously recorded from the type locality in northern Vietnam, and several localities in northern and central Laos (Smith 1896;Fischer and Dautzenberg 1904;Inkhavilay et al. 2019). Recently, we recorded this species from two localities in Nan Province, northern Thailand.
Remarks. Smith (1896: 128) introduced two nominal subspecies of H. distincta from northern Vietnam, which were distinguished on the basis of shell color. The "var. funerea" has a purplish-black tinted shell (Fig. 5B), while the "var. pallidior" possesses a yellowish or olive-yellow shell (Fig. 5A). Since then, no new specimens of these two color forms have been critically examined or their status verified. We have collected both living snails and empty shells of these two forms in recent surveys in Laos and Thailand. The two forms are anatomically and genetically identical, and occur syntopically, and thus, in our judgement, are examples of different shell colors of the same species. We recognize H. funerea as a distinct and valid species, and treat Nanina distincta var. pallidior Smith, 1896 as its junior synonym (ICZN 1999: Art. 24, 74).
Hemiplecta funerea can be distinguished from H. distincta by the angulated, dark brown or yellowish shell, distinct penial sculpture, and a long and distinctively coiled epiphallic caecum (Fig. 6A) compared to the short and straight epiphallic caecum in H. distincta (Fig. 4A). The blackish reticulated skin on a yellowish background and blackish eye tentacles contrast with the greyish body in H. distincta (Fig. 1B-E). Shell. Shell relatively small (height up to 25 mm, width up to 35 mm), elevated to slightly depressed, upper surface with distinct nodules arranged on growth line, and lower shell surface nearly smooth. Last whorl keeled; aperture large and ovate; lip simple to slightly expanded and dark brown. Umbilicus widely opened and deep (Fig. 5C).

Hemiplecta esculenta
Genitalia. Genital tracts similar to those of H. humphreysiana (Fig. 7A). Internal wall of penis with sculpture encircling penial verge. Penial sculpture (ps) consists of small to large papillary knobs arranged in oblique lines on penial wall; relatively smaller knobs surrounding penial verge. Penial verge (pv) small, short, conic, and with smooth surface (Fig. 7B). Gametolytic sac (gs) bulbous with undifferentiated duct. Internal wall of vagina with series of thin longitudinal vaginal pilasters (vp). Dart apparatus (da) relatively short; internal wall of chamber with smooth wall, and papilla of dart apparatus (dp) slightly elongate, conic, and with smooth surface (Fig. 7B).
External features. Living snail exhibits similar soft body morphology, pulmonary cavity and caudal structure (Fig. 7D) to that of H. humphreysiana. The distinct characters are the brownish to greyish body and mantle edge; right and left shell lobes absent (Fig. 1F).
Distribution. Previously known only from the type locality in northern Vietnam (Maassen 2006) and Xieng Khaung, northeastern Laos (Inkhavilay et al. 2019). Recently, we have located populations from northern Thailand in Chiang Mai Province.
Remarks. The shell features were carefully described in Maassen (2006). The original description of H. esculenta was based on seven shells and placement within Hemiplecta was provisional (Maassen 2006), and none of the topotypic specimens have subsequently been examined. The samples from Thailand show only minor variations from the type series, in the presence of a narrow brownish spiral band and slightly elevated spire, which we attribute to intraspecific variation. It is important to examine the genitalia of the topotypic material.
Hemiplecta nemorosa sp. nov. http://zoobank.org/5B542C60-C447-40B9-BC1D-AD24774C4AFD Figures 8-10J Etymology. The species name is derived from the Latin word "nemoris" meaning "full of woods or shady," which refers to the type locality of this new species in the dense deciduous forest.
Description. Shell medium sized (height up to 15 mm, width up to 45 mm), depressed conic, thin and dextral. Whorls 5 to 6, increasing regularly, slightly convex, with very wide and shallow suture. Spire convex; apex acute; embryonic shell smooth; following whorls with thin growth lines and radial wrinkles or undulating surfaces. Periostracum thin and transparent. Shell pale brownish to yellowish. Last whorl angular with strong peripheral keel which is much reduced near aperture. Aperture not descending, widely ovate and moderately oblique; lip simple to slightly thickened in adult specimen. Columella slightly dilated; parietal callus slightly thick and translu- cent. Umbilicus narrowly opened, deep, and partly covered by reflected columellar lip (Fig. 8).
Genitalia. Atrium (at) long. Penis (p) long slender, cylindrical, and encircled by thick penial sheath (psh) extending to about half of penis length. Epiphallic caecum (ec) short, straight; penial retractor muscle (pr) thin and attached to the tip. Epiphallus (e) short, about half of penis length. Flagellum (fl) short, stout, and with thin muscle bands connected to penial sheath. Vas deferens (vd) small tube (Fig. 9A). Internal wall of penis with sculpture over entire chamber with uniform scale-like or triangular lingulate pilasters varying in size from small to large and pilasters encircling penial verge smaller than in the middle of chamber. Penial verge (pv) small, conic, and with smooth surface (Fig. 9B). Vagina (v) long, cylindrical, about same length as penis; internal wall with thin and smooth longitudinal vaginal pilasters (vp). Dart apparatus (da) short and enlarged muscular cylinder; externally covered with thin longitudinal muscular bands around half of dart apparatus length. Internally with irregular wall, dart papilla (dp) conic and smooth. Gametolytic sac (gs) bulbous without distinct duct. Free oviduct (fo) long and encircled with thin blackish muscular tissue. Oviduct (ov) long and with lobules; prostate gland bound to oviduct. Albumen gland, hermaphroditic duct, and hermaphroditic gland missing from the examined specimen (Fig. 9A, B).
Distribution. This new species is currently known only from the type locality in northern Thailand. Figure 10. Representative SEM images of radula A-C Hemiplecta humphreysiana, specimen CUMZ 4573 from Singapore A central and lateral teeth B transition from lateral teeth to marginal teeth and C outermost marginal teeth D, E Hemiplecta distincta, specimen CUMZ 4560 from Chanthaburi, Thailand D central and lateral teeth and E outermost marginal teeth F, G Hemiplecta funerea, specimen CUMZ 4575 from Nan, Thailand F central and lateral teeth and G outermost marginal teeth H, I Hemiplecta esculenta, specimen CUMZ 4553 from Chiang Mai, Thailand H central and lateral teeth I transition from lateral teeth to marginal teeth J Hemiplecta nemorosa sp. nov., paratype CUMZ 5253 from Maehongsorn, Thailand. Central tooth indicated by 'C'. Numbers indicate the tooth order from lateral to marginal end.

Remarks.
The shell morphology of this new species is similar to H. uter (Theobald, 1859) from Myanmar and Falsiplecta integripedia Schileyko & Semenyuk, 2018 from southern Vietnam. This new species, however, differs by having a shell width almost two-times larger than H. uter, but further comparison of anatomical characters is necessary to confirm their distinction. Hemiplecta nemorosa sp. nov. clearly differs from F. integripedia in having a well-developed dart apparatus, globular gametolytic sac, and long epiphallus and flagellum. In contrast, F. integripedia has no dart apparatus, a long gametolytic duct, a very short epiphallus and the vas deferens attached near the tip of the epiphallus (flagellum lacking). Hemiplecta nemorosa sp. nov. also differs from H. undosa (Blanford, 1865) by having a relatively smaller shell size, an angular last whorl with strong peripheral keel, and a narrow umbilicus. In contrast, H. undosa has a rounded to slightly shouldered last whorl, and a wide and deep umbilicus.

Species list from Indochina including Peninsular Malaysia and Myanmar
This synoptic list includes all the nominal species-group names that have been attributed to Hemiplecta s.l. and have the type locality within the geographic area covered by mainland Indochina, Peninsular Malaysia or the southeastern part of Myanmar. All the nominal species group names are listed alphabetically where their original combinations and original publication were provided. In nearly all instances, the original literature was checked for authorship and date, page numbers of the original description and illustrations, and type locality to ensure accuracy of the entries. The usage of the nominal name, necessary references that provided descriptions or images of possible type specimens, and recent taxonomic treatment articles that placed species into the genus Hemiplecta are also listed. The current taxonomic status (validity or synonymy) of each taxon is provided, mainly following recent literature and this study. The depository information of the name-bearing types (holotype, lectotype, or syntype) is provided. The name-bearing types are illustrated when possible; exceptions are those recently published in Inkhavilay et al. (2019), Páll-Gergely (2019), and . However, in the cases where the name-bearing types could not be traced, topotypic or authentic reference specimens are illustrated instead for further comparison. In some instances, information about the authorship, type series, and type locality is discussed under the remarks section. The type specimens were located (preserved) in several museums, as follows:

Current taxonomic status. Hemiplecta. Valid species.
Type specimens. The type specimens could not be traced. Remarks. The manuscript name "cymatium Bens." was never published by Benson. It was first mentioned in the species list published by Pfeiffer (1855: 121), but without any indication to make the name available (ICZN 1999: Art. 12). Later, Pfeiffer (1856b) published this name with a description and illustration and attributed it to Benson. However, since Benson did not write the description, the authorship of this taxon should be attributed to Pfeiffer, who formally described it and made the name available.
The original description includes an illustration and one set of shell measurements. The type series of the taxa could not be traced in the UMZC and NHM collections. There are three specimens from UMZC I.104350 ex. Benson collection accompanied by a label with the taxon name but without collection locality. A specimen that closely matched the original description is figured herein (Fig. 11B).
Remarks. The number of specimens was not clearly stated and only one set of shell measurements was given in the original description. The single specimen from the type lot is illustrated herein for the first time.

distincta (Pfeiffer, 1850)
Helix distincta Pfeiffer, 1850: 69, 70 Remarks. This species was described based on specimens from the Cuming collection. The original description did not include an illustration and only one set of shell measurements was given. Later, Pfeiffer (1853) re-published the description and figured this species based on material from the Cuming collection. The NHM collections contain a lot of three shells from the Cuming collection. The original label, not in Pfeiffer's handwriting, states the taxon name and gives the collection locality as "Hab. Moluccas (Pfr. Zeitschr. 1850. p. 69)", and a small printed label stating "Type?". Additionally, the collection localities "Siam & Camboja" and "Siam & Cochin Chine (Martens)" were probably added at a later date. Therefore, we consider this lot to be possible syntypes. The specimen that closely matched the measurements in the original description and illustration in Pfeiffer (1853) is figured herein.
The museum collection and current published record with detailed geographical data of H. distincta are only from Indochina. Therefore, the type locality "Insulis Moluccis [Molucca Islands in eastern Indonesia]" possibly error or mislabeling.
Remarks. The original description included illustrations and one set of shell measurements. However, the species description was not explicitly based on one specimen. There are two shells in the NHM type lot with an original label stating "Types", subsequently changed to read "holotype red spot". The shell that matched the measurements given in the original description and that has a red spot in the aperture is figured herein.
Remarks. This species seems to be described based on a single specimen and the author refers to the diagnosed character "two broad shallow spiral grooves situated near the periphery of body whorl at dorsal side". However, using just one character without any further independent diagnostic characters could raise doubt about the taxonomic status. The single shell may reflect an abnormality during the growth stage and, apart from this trait, all the other shell characters all are within the range of morphological variations seen within H. distincta. Therefore, we consider H. franzhuberi as a junior synonym of the more common and widespread H. distincta.
It would be very useful if this new species were compared with sympatric or geographically proximate species (i.e. H. distincta or H. pluto) instead of the distant species H. abbasi Maassen, 2009 from Sumatra (Maassen 2009 Remarks. The species description is clearly based on more than one specimen. The original description does not include an illustration, and measurements of the largest specimen are given. The NHM collection contains a lot of a single specimen with a label stating "var. funerea". This specimen matched well with the original description and is here designated as the lectotype to stabilize the name. Inkhavilay et al. (2019: fig. 35f ) state this specimen is the syntype of "var. funerea" but wrongly apply the images of "var. pallidior" instead.
Remarks. The topotype specimen from Godwin-Austen collection NHMUK 1903.7.1.309 (1 shell; Fig. 12D) with collection locality from Needoung Thoung, Ataran valley, Tenasserim is figured herein. Remarks. Páll-Gergely et al. (2020) attributed the diagnostic character of this name as a morphological variation of the widely distributed H. pluto, so this is treated as a junior synonym.

huberi Thach, 2017
Helminthoglypta huberi Thach, 2017: 54, figs ; Fig. 12E) from Pondicherry. Remarks. This species was clearly described based on more than one specimen. The author's description clearly indicates that the type series was from two collection localities: "Pondicherry" received from Mr. Humphreys and "Singapore" received from Mr. Balastire. Later, Lea (1841) re-described the species and illustrated a single specimen. The Smithsonian collections contain a lot of a single shell from the Lea collection as from "Pondicherry". This specimen matched well with the illustration and the measurements given in the original description.
The records of this species from "Pondicherry" [the historical name probably referred to the cities on the east coast of India] have never been verified. Currently, the genus Hemiplecta are distributed from Southeast Asia to Southeast Asia and New Guinea, except one species recorded from the Maldives (Schileyko 2002). Therefore, "Pondicherry" is probably an erroneous record (see also Godwin-Austen 1898: 74), and "Singapore" is possibly the correct type locality of this species.

jensi Páll-Gergely, 2019
Hemiplecta jensi Páll-Gergely, 2019: 86-88, figs 1-6. Type locality: Vietnam, Thanh Hoa Province, Pu Luong N.R., surroundings of Village Am.  (2020); we agree with their decision. An image of a living specimen in the original description (Thach 2018: fig. 888) shows an aulacopod type of pedal groove, whereas Camaenidae has a holopod type of pedal groove (see Solem 1959: fig. 2). The assignment of this species to the genus Hemiplecta is most likely, due to the helicarionoid snails having a relatively large shell size, simple apertural lip (slightly thickened), and narrow umbilicus (Schileyko 2002). Anatomical examination will help elucidate the appropriate generic position of this species.
Thach (2018) mentioned depositing the holotype at the Field Museum of Natural History, Chicago. However, the holotype has not arrived at the FMNH collection (Jochen Gerber, personal communication on October 2020).
Remarks. This species was originally proposed as a junior secondary homonym from the same locality as the senior homonym. Basically, this junior homonym agrees well in all diagnostic shell characters of a red-brown shell, strong peripheral keel, and shell shape that all lie within the range of variation of the present species. This species is synonymized with H. khamducensis (Thach & Huber, 2018) herein, and, therefore, the replacement name is not necessary at present. Thach (2020) stated that the holotype was deposited at the Natural History Museum in London. However, the holotype has not arrived in the NHM collection (Jonathan Ablett, personal communication on May 2021). Remarks. Only one specimen in the syntype lot and the spire was broken after the original description. 20 nemorosa sp. nov.
Remarks. The species is described herein (see systematic part).

neptunus Pfeiffer, 1861
Helix neptunus Pfeiffer, 1861a: 190.  Remarks. This name was described based on specimens from the Cuming ex. Mouhot collection. The original description did not include illustrations, and only one set of measurements was given. Later, Pfeiffer (1861b) re-described and illustrated a single specimen from the Cuming collection. There are two specimens from the Cuming collection in the NHM type lot with an original label in Pfeiffer's handwriting stating the species name, collection locality and "pernobilis Fer. Var.? and pl.74, f. 4.". The specimen that corresponded to the shell measurements in the original description and illustration in Pfeiffer (1861b)  Remarks. The number of specimens was not clearly stated, and only one set of shell measurements was given in the original description. The single specimen from the type lot is illustrated herein for the first time.

platytaenia Möllendorff, 1900
Hemiplecta platytaenia Möllendorff, 1900: 121 Remarks. The number of specimens was not clearly stated, and only one set of shell measurements was given in the original description. The single specimen from the type lot is illustrated herein for the first time. Möllendorff (1900: 121) also stated that this species differed from H. neptunus (=H. distincta) in having a flatter shape, with a weak keel, and a spiral band on periphery. Remarks. Pfeiffer (1863a) stated that this species was described based on specimens from the Cuming collection. The original description did not include an illustration, and only one set of shell measurements was given. There are two specimens in the mixed type-lot of different species. The specimen that had an original label in Pfeiffer's handwriting states "H. pluto Pfr." and the collection locality "Lao Mountains, Camboja". The specimen that matched the description and shell measurements given in the original description, and the illustration in Pfeiffer (1863b: pl. 55, figs 8, 9) is here designated as the lectotype to stabilize the name.
The other shell from the same collection lot with label stating "var. neptunus young" was identified as H. distincta. This specimen is not part of the type series and, therefore, excluded from this designation. Pfeiffer (1863a) described this species based on the specimen collected by H. Mouhot in the Cuming collection, and "Lao Mountains, Camboja" is the type locality. We have seen specimens with more precise geographical location from the Khammouan Province to Luang Prabang Province, Laos (Inkhavilay et al. 2019).

theodori (Philippi, 1846)
Helix theodori Philippi, 1846: 191, 192 Remarks. The original description did not include an illustration, and only one set of shell measurements was given. The author stated, "All the specimens", implying that this description was based on more than one specimen. Blanford (1865) also mentioned "? horny when fresh" and "…dead and bleached" in the original description. The NHM collection contains a lot of two bleached shells from the Godwin-Austen ex. Blanford collection with an original label stating the species name; however, this is probably not in Blanford's handwriting. Therefore, we consider this lot to be a probable syntype. However, the specimen that matched well with the original description and shell dimensions is figured herein.

uter (Theobald, 1859)
Helix uter Theobald, 1859: 305.  Remarks. The original description clearly states that this taxon was described based on only one specimen collected by W.S. Atkinson (Theobald 1859). The NHM registration records show that a specimen was purchased from W. Theobald with the label stating "type" and locality given as "Moulmein". Therefore, this single specimen is recognized as the holotype fixed by monotypy (ICZN 1999: Art. 73 Remarks. The original description did not contain any illustrations, and only one set of measurements was given. Godwin-Austen stated that the type series was from his own and Theobald's collections. The NHM collection contains two lots that are considered to constitute the type series. Lot NHMUK 1903.7.1.2108 consists of a single specimen from the Godwin-Austen ex. Theobald collection and has original labels giving the species name "zimmayensis" and type collection locality "Siam". The other lot consists of a single shell, NHMUK 1888.12.4.2007, and the NHM registration book shows that this specimen lot was purchased from W. Theobald, and with an original label stating the species name "H. Zimmayensis G.A." and type collection locality "Zimmay territory (Siam)". This specimen (NHMUK 1903.7 Remarks. The type specimen of this nominal species seems to be based on the immature shell, and the genital organ was not examined. However, the molecular phylogeny based on the juvenile specimens from approximate type locality strongly suggests it is a member of the genus Hemiplecta ).
Remarks. The original description does not include any illustration, and the author clearly stated that measurements given were based on two specimens. There is a specimen lot in the NHM ex. Annandale and Robinson collection consisting of two shells; one of these has a malformed shell form as stated in the original description. These two shells are considered as the syntypes and figured herein for the first time.
A recent phylogenetic study has recognized this taxon as a junior synonym of H. salangana, a widespread species in the southern peninsula of Thailand and northern Peninsular Malaysia . Remarks. The type specimen was recently figured, and recent systematic revision based on both genitalia morphology and DNA phylogeny confirm their generic status within the genus Hemiplecta   : Laidlaw 1931: 191. Berry 1963 fig. 61. Hemiplecta salangana: Sutcharit et al. 2021: 205, 206, fig. 3c, d.
Remarks. The type specimen was recently published.

Results and conclusion
Nine valid species of the genus Hemiplecta occur in Thailand, five of these are the dextral shell coiling species, and the other four are sinistral shell coiling species. In order to broaden our comparison among species of Hemiplecta s.l., we gathered and compared anatomical data from the literature for nineteen species (Table 2). This comparison indicated that fourteen species are likely to have been correctly placed in the genus Hemiplecta, but not the other five species. The genital characters are relatively similar among species within this genus, except for the penial verge, penial sculpture, and terminal part of male genitalia (epiphallus, epiphallic caecum and flagellum), which are taxonomically informative at the species level (Table 2). Interestingly, these fourteen species all have a similar gametolytic organ structure: globular gametolytic sac with an undifferentiated duct. A recent systematic study has shown congruence between the traditional morphology-based species taxonomy and the molecular phylogeny . This indicates that accurate generic recognition can be based on the genitalia character especially the globular shape of the gametolytic sac. However, the critical role of the gametolytic organ other than extracellular digestion of excess reproductive products has never been reported in the stylommatophoran (Goméz 2001;Baur 2010).
This comparison further indicated that the other five species are likely to have been inappropriately placed in the Hemiplecta (Table 2). Four species: H. densa (Adams & Reeve, 1850) from the Philippines, H. werberi (Sarasin & Sarasin, 1899) from Sulawesi, H. foersteri Kobelt, 1914 from Papua New Guinea, andH. belerang Cilia &Abbas, 2012 from Sumatra exhibit a long gametolytic duct, with or without a dart apparatus (Table 2) and are clearly distinct from the typical characteristics of the Hemiplecta (Wiegmann 1898;Niethammer 1937;Wiktor 2003;Cilia and Abbas 2012). On the other hand, these four species have a dextral shell, and the presence of shell lobes suggest a close relationship to the genus Nanina. In addition, H. malaouyi (Morgan, 1885) from Peninsular Malaysia exhibits a coiled epiphallic caecum, long gametolytic organ, and presence of shell lobes (Table 2), which are the unique characteristics of the Macrochlamydinae (Blanford and Godwin-Austen 1908;Solem 1966;Schileyko 2003). However, the relatively large shell size (width about 40 to 60 mm) is distinct from other known genera within the Macrochlamydinae. Further anatomical information and molecular analyses will elucidate whether the generic placement is appropriate or whether these species form a distinct group.
In the species list, 39 available species-level names are recognized as part of the genus Hemiplecta and described from Indochina, including Peninsular Malaysia and Myanmar. There are six nominal species for which the name-bearing type could not be discovered, except the four nominal species: H. auriettae, H. gordonae, H. textrina and H. theodori where the topotypic specimen are figured as representative. However, generic placement of many species are still provisional because these species are known only from their shell descriptions without the genitalia characters. Like the other land snail group in Indochina, a systematic revision has never been studied, and species recognition is difficult. The species have long been described with only a brief description and without illustrations of unique characters of the species. This species list with illustrated type or authentic specimens provides a key species data and facilitates proper species identification.
lalongkorn University for their kind help during field trips; also T. Asami, A. Wiwegweaw, and S. Natsupakpong for providing some important literature. The work was mainly supported by TRF Strategic Basic Research DBG 6080011 (2017-2019), The Thailand Research Fund (TRF-DPG628001), and Center of Excellence on Biodiversity (BDC-PG4-163008). Additional supports were from Grants for Development of New Faculty Staff, Chulalongkorn University, the BRT-R 252108, the RES-A1B1-7, the CHE-RES-RG, the SP2-TKK2555-PERFECTA and the Darwin Initiative Project (no. 14-653). We also express our gratitude for the comments from anonymous reviewers that have greatly improved the manuscript.