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The snorkel snail genus Rhiostoma Benson, 1860 is comprised of terrestrial cyclophorid snails with wide-ranging species diversity and radiation in Southeast Asia. The typical characters of the genus are a depressed shell, a detached and descending portion of the last whorl with a distinctive peristomal breathing device attached, and a calcareous cup-shaped operculum. Herein, we have revised the systematics of extant species based on shell morphology combined with COI barcoding. From these thirty recognised species, twelve are described as new to science: R. ? amarapuraensesp. nov., R. anceyisp. nov., R. breviocollarsp. nov., R. ebenozosterasp. nov., R. cheliopegmasp. nov., R. furfurosumsp. nov., R. gnomus, sp. nov., R. lannaensesp. nov., R. laoensesp. nov., R. platymorphasp. nov., R. rhothonotaphrosasp. nov., and R. tigrinasp. nov. All conchological characters are provided via illustrations of type specimens and living snails, and descriptions of the shells and radulae. Phylogenetic analysis based on partial COI gene sequences strongly supports the designated morphospecies and a monophyletic Rhiostoma, confirming that all pterocyclinid snails with a calcareous, cup-shaped operculum belong to the same clade. A high intra-specific divergence was observed in R. jalorensis and R. housei populations from locations in close proximity, suggesting a lower dispersal and higher level of isolation. The low inter-specific divergence found in R. hainesi, R. samuiense, R. asiphon, and R. rhothonotaphrosasp. nov. supports their recent diversification and local adaptation, and is congruent with their marked morphological differences. Finally, nine formerly Rhiostoma-placed species were reclassified into either the genus Cyclotus or the genus Opisthoporus.

Namkongnaia inkhavilayi gen. et sp. nov. urn:lsid:zoobank.org:act: 622F7788-F0A4-449D-814A-5B49CD20B228 Figs. 3, 4A–B; Tables 1, 3 Pilsbryoconcha lemeslei – Brandt 1974: 263, pl. 18 fig. 22 (in part, only records from “Nang Rong (Prov. Burirum)”, Thailand) [non Anodonta lemeslei Morelet, 1875]. Etymology This new species is dedicated to our colleague and distinguished malacologist, Dr Khamla Inkhavilay, from the National University of Laos. Type material Holotype LAOS • Kammoune Province, Bunghona Market, 7 km N of Xe Bangfai River; 17.13674° N, 104.98591° E; E. Jeratthitikul, K. Wisittikoson, A. Fanka, N. Wutthituntisil and P. Prasankok leg.; sold by local people; MUMNH-UNI2831. Paratypes LAOS • 5 shells; same collection data as for holotype; MUMNH-UNI2832 to UNI2835. Other material LAOS • 2 shells; Nong Njang, N of Vientiane; R. Brandt leg.; BMNH-MP-D262 • 1 shell; Nong Bua Thong near Vientiane; R. Brandt leg.; ANSP-H19043 • 2 shells; Nong Bua Thong near Vientiane; R. Brandt leg.; MCZ-280932 • 2 shells; Nong Bua Thong near Vientiane; R. Brandt leg.; SMF-258771 • 2 shells; Nong Bua Thong, Vientiane; R. Brandt leg.; SMF-319294 • 1 shell; Bang Jian near Vientiane; R. Brandt leg.; ANSP-H19044 • 6 shells; Nang Njang near Vientiane; MNHN-MP-3154 • 4 shells; Nang Njang near Vientiane; R. Brandt leg.; MNHN-MP-3157 • 3 shells; swamp near Pakse; R. Brandt leg.; SMF-225743. THAILAND • 1 shell; Nakhon Phanom, Tha Uthen, Tha Uthen, Thuai River; 17.56221° N, 104.60890° E; E. Jeratthitikul, K. Wisittikoson and P. Prasankok leg.; MUMNH-UNI2706 • 1 shell; Sakon Nakhon, Sawang Daen Din, Bong Tai, Songkhram River; 17.39570°N, 103.30048°E; E. Jeratthitikul and K. Wisittikoson leg.; MUMNH-UNI0318 • 1 shell; Buriram, Nang Rong, Huai Tadjek; R. Brandt leg.; USNM-786219 • 1 shell; Buriram, Nang Rong, Huai Tadjek; R. Brandt leg.; SMF-220822 • 4 shells; Buriram, Nang Rong; SMRL-2662 • 1 shell; Buriram, Huai Ta Djek near Nang Rong; ANSP-H19042. Description Shell medium-sized, length 80.9–102.2 mm, height 26.2–37.2 mm, width 13.6–19.1 mm (Table 3). Shell rather thin, narrow and elongated (H/L ratio = 0.32–0.36), very inequilateral, compressed. Dorsal margin straight, anterior at the same level as posterior. Umbonal area eroded, not elevated. Anterior margin round; posterior margin elongated and rounded. Ventral margin almost straight, slightly curved upward posteriorly. Posterior ridges low, wide and obtuse, not prominent. Periostracum thin, greenish to dark brown, the eroded part coppery-brown. Shell surface with fine growth lines. Ligament very narrow. Hinge without dentition, posterior end of the hinge structure with V-shaped fossette. Anterior adductor muscle scar placed relatively far from the dorsal margin, shallow, ovate, fused with pedal retractor muscle scars; posterior adductor muscle scars very shallow, almost invisible. Pallial line very faint. Nacre whitish with cream tint near the umbo. Excurrent aperture smooth, shorter than incurrent. Incurrent with 1–2 rows of conical papillae, varying in length. Small epithelial folds form a fused bridge separating excurrent and incurrent aperture. Gills elongated and slightly ribbed. Anterior margin of inner gills slightly longer and wider than that of inner outer gills. Glochidia unknown. Distribution This new species appears to occur in the Lower Mekong watershed. In Laos, it was recorded from several locations near Vientiane, Kammoune, and Champasak Provinces. In Thailand, it is known from the Songkram and Mun river basins in the northeast region (Brandt 1974). Habitat Specimens of the new species were bought on a local market near the Xe Bangfai River. However, specimens from the Thuai and Songkhram Rivers in Thailand were found buried in the mud substrate of still water. Brandt (1974) also reported ponds as a general habitat of this species. Remarks The specimens recorded by Brandt (1974) from Nang Rong, Buriram, Thailand (SMF-220822) show a shorter but wider shell and a relative rounded ventral margin. In addition, although the hinge area of this species has no dentition, the trace of a rudimentary pseudocardinal tooth is seen as a very tiny tubercle in some individuals.

Genus Namkongnaia gen. nov. urn:lsid:zoobank.org:act: 77AD43CF-83EC-42B8-B021-A950D4E701A0 Type species Namkongnaia inkhavilayi gen. et sp. nov., by present designation. Differential diagnosis The new genus is distinguished from other genera in Pseudodontini by having a narrow, elongated, and less inflated shell. It is also represented as a distinct clade in multi-locus phylogenetic analyses. Description Shell Medium-sized, thin, narrow and elongated, rather compressed, very inequilateral; anteriorly round; posteriorly round or somewhat pointed; umbonal area not elevated and usually eroded. Ligament very narrow. Hinge without dentition, posterior end of the hinge structure with V-shaped fossette. Anterior adductor muscle scar shallow, ovate, and fused with pedal retractor muscle scars; posterior adductor muscle scar very shallow. Excurrent aperture smooth, shorter than incurrent; incurrent with 1-2 rows of conical papillae. Gills elongated and slightly ribbed; anterior margin of inner gills slightly longer and wider than outer gills. Etymology The generic name “ Namkongnaia ” is from the word “Namkong”, a name for the Mekong River used by Thai and Lao peoples, and Greek word “naiad” meaning freshwater mussels. The name of this genus thus means “freshwater mussels from Mekong River”. Distribution Mekong River and its tributaries in Laos, Cambodia, and Thailand (Fig. 3; Brandt 1974; Ng et al. 2020). The distribution range may include the Mae Klong River Basin in Thailand (Brandt 1974), although the taxonomic status of the specimens recorded in Brandt (1974) needs to be confirmed. Remarks The new genus is currently composed of two species, which are confirmed by our multi-locus molecular data. Other potential species are probably among those previously recognized under the name ‘ Pilsbryoconcha lemeslei ’ by Brandt (1974). This includes the specimens from ‘Bang Pae’, Mae Klong River Basin, Ratchaburi Province (USNM-786217, SMF-BR2663, and ANSP-H19041). These specimens show elongate and narrow shell, which are diagnostic characters of the genus, but differ from other species by having a minute concave ventral margin, and truncated posterior margin with a pointed posterior end. Further examination of fresh materials and DNA investigation would confirm the taxonomic status of these populations., Published as part of Jeratthitikul, Ekgachai, Sutcharit, Chirasak, Ngor, Peng Bun & Prasankok, Pongpun, 2021, Molecular phylogeny reveals a new genus of freshwater mussels from the Mekong River Basin (Bivalvia: Unionidae), pp. 119-142 in European Journal of Taxonomy 775 on pages 128-129, DOI: 10.5852/ejt.2021.775.1553, http://zenodo.org/record/5593519, {"references":["Brandt R. A. M. 1974. The non-marine aquatic Mollusca of Thailand. Archiv fur Molluskenkunde 105: 1 - 423.","Ng T. H., Jeratthitikul E., Sutcharit C., Chhuoy S., Pin K., Pholyotha A., Siriwut W., Srisonchai R., Hogan Z. S. & Ngor P. B. 2020. Annotated checklist of freshwater molluscs from the largest freshwater lake in Southeast Asia. ZooKeys 958: 107 - 141. https: // doi. org / 10.3897 / zookeys. 958.53865"]}

Namkongnaia lemeslei (Morelet, 1875) gen. et comb. nov. Figs 3, 5; Table 1 Anodonta lemeslei Morelet, 1875: 328, pl.14 fig. 1. Pilsbryoconcha lemsleyi [sic] Simpson, 1900: 558. Anodonta lemeslei ��� Crosse & Fischer 1876: 333. ��� Morlet 1889: 167. Pilsbryoconcha lemeslei ��� Simpson 1914: 244. ��� Haas 1920: 300, pl. 37 fig. 4.; 1969a: 382. ��� Brandt 1974: 263 (in part, only records from ���Aranyapratet (Prov. Prachin Buri)���, Thailand). ��� Graf & Cummings 2007: 311. ��� Zieritz et al. 2018: 29���44. ��� Ng et al. 2020: 122, fig. 3j.; ��� Graf & Cummings 2021a: 22. Type material Syntype CAMBODIA ��� 2 shells; ���Cambodge, Marecages de Battambang ���; Le Mesle leg.; MNHN-MP-3150. Other material CAMBODIA ��� 6 shells; Siem Reap, Chi Kraeng, Kampong Kdei, Kampong Kdei River; 13.13279�� N, 104.33989��E; E. Jeratthitikul, C. Sutcharit, W. Siriwut, S. Chhuoy and T.H. Ng leg.; MUMNH-UNI2825 to UNI 2829, UNI 2669 ��� 5 shells; Grand Lacs; MNHN-MP-3156 ��� 2 shells; Phnom Penh; MNHN- MP-3158 ��� 4 shells; Cambodge; MNHN-MP-3163, MNHN-MP-3171. Differential diagnosis This species resembles the type species, but it can be distinguished by the relatively smaller size with a shell length of 67.0���81.0 mm (vs 80.9���102.2 mm), anterior adductor muscle scar placed near the dorsal margin (vs relatively far from the dorsal margin), adductor muscle scar relatively deep for the thin shell (vs shallower, although thicker shell), nacre yellowish (vs whitish), anterior margin rounded and little shouldered (vs wider and not shouldered), and the anterior portion of the dorsal margin somewhat lower than the posterior (vs at the same level in both anterior and posterior portions). Description Shell medium-sized, rather thin, narrow and elongated (H/L ratio = 0.39), very inequilateral, compressed. Dorsal margin straight; somewhat lower anteriorly than posteriorly. Umbonal area eroded, not elevated. Anterior margin round, little shouldered; posterior margin elongated, somewhat pointed. Ventral margin almost straight, or minutely concave in the middle in old specimens, posteriorly slightly curved upward. Posterior ridges low, wide and obtuse, not prominent. Periostracum thin, yellowish to dark brown, the eroded part coppery-brown. Shell surface with fine growth lines. Ligament very narrow. Hinge without dentition, posterior end of the hinge structure with V-shaped fossette. Anterior adductor muscle scar placed near the dorsal margin, relatively deep for the thin shell, ovate, fused with pedal retractor muscle scars; posterior adductor muscle scars very shallow, almost invisible. Pallial line very faint. Nacre yellowish with creamy tint near the umbo. Additional description Examination of the soft body of newly collected specimens in this study yielded additional description of the animal: excurrent aperture smooth, shorter than incurrent; incurrent with 1���2 rows of conical papillae, similar in length; small epithelial folds form a fused bridge separating excurrent and incurrent aperture; gills elongated and slightly ribbed; anterior margin of inner gills slightly longer and wider than that of outer gills. However, no brooding specimens were available to examine glochidia. Distribution This species is restricted to Tonle Sap Lake and its tributaries in Cambodia. Its distribution probably reaches the headwater of Tonle Sap basin in eastern Thailand (Brandt 1974). Habitat This species was recorded in mud substrate in still sections of rivers or in lentic habitat, i.e., ponds and lakes (Morelet 1875; Morlet 1889; Brandt 1974). Remarks This species has been noted for its rarity (Brandt 1974; Ng et al. 2020). Only a few lots are available in museum collections. We obtained additional fresh materials from the Kampong Kdei River, Siem Reap Province. These specimens resemble the syntype MNHN-MP-3150 (Fig. 5D). The only detected variations are less erosion of the umbo area and more yellowish-brown colour of the periostracum. Morelet (1875) described Anodonta lemeslei based on specimens collected by M. Le Mesle. The figured specimen is eroded, with a concave ventral margin (Fig. 5B���C), while another shell has a relatively parallel shell and a straight ventral margin (Fig. 5D). Morelet (1875) identified the ventrally concave specimens as fully-grown individuals, while the ventrally straight specimens were considered to be young individuals. The concavity trait is quite unusual for species that live in stagnant water habitats. Nevertheless, some specimens from Tonle Sap Lake (MNHN-MP-3156) also show the characteristic of a concave ventral margin. However, all of our specimens have a straight ventral margin. This raises some suspicions about the existence of both variations. In this study, we retain both variations as the same species until topotype specimens with concave ventral margin are available for DNA analysis. Apart from a ventral margin, other morphological variations are also observed. The posterior ends of some specimens are more or less rounded than others. Crosse & Fischer (1876) noted that some specimens from Cambodia are narrower and more elongated than the type figure. The posterior dorsal margin in some specimens is also somewhat higher. Morelet (1875) noted that one of his type specimens is undoubtedly similar to Anodonta schomburgki Martens, 1860 [= Pilsbryoconcha compressa (Martens, 1860)]. However, it is more or less narrow, and the high posterior cannot be considered a posterior wing as in Anodonta schomburgki Martens, 1860. Several records and specimens previously identified as Pilsbryoconcha lemeslei have been examined and listed in this species. However, a specimen from Khanh Hoa, Vietnam, is conchologically different and does not belong to this species (Thach 2007; Do et al. 2018); rather, it is probably a species of Pilsbryoconcha. This specimen shows more or less curved ventral margin and relatively wider shell. Similarly, the DNA sequence of specimen assigned to Pilsbryoconcha lemeslei by Bolotov et al. (2020) and Konopleva et al. (2021) is recovered as monophyletic within the Pilsbryoconcha, together with specimens of Pilsbryoconcha exilis from Java, Indonesia (type species) and Pilsbryoconcha linguaeformis, another sympatric species with Namkongnaia lemeslei gen. et comb. nov., from Tonle Sap Lake, in our phylogenetic analyses. However, without examination of the specimen mentioned in Bolotov et al. (2020) and Konopleva et al. (2021), it can only be classified to the genus Pilsbryoconcha., Published as part of Jeratthitikul, Ekgachai, Sutcharit, Chirasak, Ngor, Peng Bun & Prasankok, Pongpun, 2021, Molecular phylogeny reveals a new genus of freshwater mussels from the Mekong River Basin (Bivalvia: Unionidae), pp. 119-142 in European Journal of Taxonomy 775 on pages 132-135, DOI: 10.5852/ejt.2021.775.1553, http://zenodo.org/record/5593519, {"references":["Morelet A. 1875. Series conchyliologiques: Comprenant l'enumeration de Mollusques terrestres et fluviatiles recueillis pendant le cours de differents voyages ainsi que la description de plusieurs especes nouvelles 4. Klincksieck, Paris. Available from https: // www. biodiversitylibrary. org / page / 11913380 [accessed 12 May 2021].","Crosse H. & Fischer P. 1876. Mollusques fluviatiles, recueillis au Cambodge, par la mission scientifique francaise de 1873. Journal de Conchyliologie 24: 313 - 342. Available from https: // www. biodiversitylibrary. org / page / 16071628 [accessed 12 May 2021].","Morlet L. 1889. Catalogue des coquilles recueillies, par M. Pavie, dans le Cambodge et le Royaume de Siam, et description d'especes nouvelles. Journal de Conchyliologie 37 (2): 121 - 199. Available from https: // www. biodiversitylibrary. org / page / 16015483 [accessed 12 May 2021].","Haas F. 1920. Die Unioniden. In: Kuster H. C. (ed.) Systematisches Conchylien-Cabinet von Martini und Chemnitz 9 (pt. 2, h. 52): 289 - 304. Available from https: // www. biodiversitylibrary. org / page / 34419468 [accessed 12 May 2021].","Brandt R. A. M. 1974. The non-marine aquatic Mollusca of Thailand. Archiv fur Molluskenkunde 105: 1 - 423.","Graf D. L. & Cummings K. S. 2007. Review of the systematics and global diversity of freshwater mussel species (Bivalvia: Unionoida). Journal of Molluscan Studies 73 (4): 291 - 314. https: // doi. org / 10.1093 / mollus / eym 029","Zieritz A., Bogan A. E., Froufe E., Klishko O., Kondo T., Kovitvadhi U., Kovitvadhi S., Lee J. H., Lopes- Lima M., Pfeiffer J. M., Sousa R., Van Do T., Vikhrev I. & Zanatta D. T. 2018. Diversity, biogeography and conservation of freshwater mussels (Bivalvia: Unionida) in East and Southeast Asia. Hydrobiologia 810 (1): 29 - 44. https: // doi. org / 10.1007 / s 10750 - 017 - 3104 - 8","Ng T. H., Jeratthitikul E., Sutcharit C., Chhuoy S., Pin K., Pholyotha A., Siriwut W., Srisonchai R., Hogan Z. S. & Ngor P. B. 2020. Annotated checklist of freshwater molluscs from the largest freshwater lake in Southeast Asia. ZooKeys 958: 107 - 141. https: // doi. org / 10.3897 / zookeys. 958.53865","Graf D. L. & Cummings K. S. 2021 a. A ' big data' approach to global freshwater mussel diversity (Bivalvia: Unionoida), with an updated checklist of genera and species. Journal of Molluscan Studies 87 (1). https: // doi. org / 10.1093 / mollus / eyaa 034","Thach N. N. 2007. Recently Collected Shells of Vietnam. L'Informatore Piceno and N. N. T, Ancona, Italy.","Do V. T., Tuan L. Q. & Bogan A. E. 2018. Freswater mussels (Bivalvia: Unionida) of Vietnam: diversity, distribution, and cobservation status. Freshwater Mollusk Biology and Conservation 21: 1 - 18. https: // doi. org / 10.31931 / fmbc. v 21 i 1.2018.1 - 18","Bolotov I. N., Konopleva E. S., Vikhrev I. V., Gofarov M. Y., Lopes-Lima M., Bogan A. E., Lunn Z., Chan N., Win T., Aksenova O. V., Tomilova A. A., Tanmuangpak K., Tumpeesuwan S. & Kondakov A. V. 2020. New freshwater mussel taxa discoveries clarify biogeographic division of Southeast Asia. Scientific Reports 10 (1): 6616. https: // doi. org / 10.1038 / s 41598 - 020 - 63612 - 5","Konopleva E. S., Bolotov I. N., Pfeiffer J. M., Vikhrev I. V., Kondakov A. V., Gofarov M. Y., Tomilova A. A., Tanmuangpak K. & Tumpeesuwan S. 2021. New freshwater mussels from two Southeast Asian genera Bineurus and Thaiconcha (Pseudodontini, Gonideinae, Unionidae). Scientific Reports 11: 8244. https: // doi. org / 10.1038 / s 41598 - 021 - 87633 - w"]}

2. Glyphidrilus kralanhensis Jirapatrasilp, Chanabun & Panha, sp. n. (Figure 3) Type specimens. Holotype CUMZ 3400 (adult; Fig. 3), paratypes CUMZ 3401 (three adults and six juveniles), and ZMH 14586 (one adult), leg. P. Jirapatrasilp, C. Sutcharit, W. Siriwut and R. Srisonchai, 6 Feb 2015. Type locality. Tapan River, Kralanh District, Siem Riep Province, Cambodia, 13º35’24.5”N, 103º24’27.8” E, 8 m amsl. (Fig. 1 Loc. 14, Fig. 4 C). Diagnosis. Wings between segment 23, 24, ½25, 25–26, 27, ½28, 29, 30, 31; clitellum in 19, 20–35, 37; genital markings paired or asymmetrical on bc in pre-wing 20, 22–24, and post-wing 26–29, 30; intestine enlarged in 16; ovaries in 13–15; spermathecae sessile, elongated oval or globular between 13/14–21/22 (Table 2). Description of holotype. Body length 107 mm, diameter 3.63 mm in segment 8, 4.06 mm in front of wings in segment 22, 2.71 mm behind wings in segment 43; body cylindrical in anterior part, quadrangular in cross-section behind clitellum. 314 segments. Body color pale brown with reddish tint from the first segment to clitellar portion. At posterior end dorsal surface broader than ventral. Clitellar wings on ventro-lateral part of clitellum, left wing in 25–30, right wing in 24–29, both 3.90 mm in height and 0.71 mm wide. Prostomium zygolobous. Dorsal pores absent. Clitellum annular but not totally fused in 20–37. Four pairs of setae per segment from 2, setal formula aa: ab: bc: cd: dd = 1.27: 0.59: 1.25: 0.74: 1.26 in segment 8 and 1.25: 0.46: 1.05: 0.52: 1.65 in postclitellar segments. Female, male and spermathecal pores not visible. Genital markings paired on bc in 23. Septa 5/6–7/8 thickest, 8/9–11/12 thick and 12/13 to the last segment thin. Gizzard globular within 8. Intestine enlarged from 16. Dorsal blood vessel anterior to 7. Hearts in 7–11. A pair of nephridia in each segment, rudimentary in 13, small in 14, and normal in 15 onwards. Seminal vesicles in 9–12. Ovaries in 13–15. Testis in 10–11. Prostate and accessory glands absent. Spermathecae sessile, elongated oval or globular in 13/14–21/22, about 0.2 mm in diameter, one to eleven on each side per segment. Variation. Body lengths of adult paratypes (4) ranged from 85–116 mm (mean ± S.D. = 100.5 ± 13.7), with 205–314 segments. Wings begin in 23, 24, ½25, or 25 and end in 26, 27, ½28, 29, 30, or 31; the most frequent position is 24–30, 31. Clitellum begins in 19 or 20 and ends in 35 or 37. All specimens have genital markings paired or asymmetrical on bc in pre-wing 23, some in 20–22, 24, and post-wing 26–29, 30. The most frequent position of spermathecae is 13/14–20/21; some occur in 21/22 (Table 2). Distribution. The new species is known only from the type locality. Habitat. The species was found on river banks where the worm casts covered the surface. The earthworms occurred in the loamy sand topsoil at about 10 cm depth. Etymology. The species was named after Kralanh district, the type locality. Remarks. Glyphidrilus kralanhensis sp. n. is the only species in the genus to contain three pairs of ovaries in 13–15 (hypergyny), and also has spermathecae in more posterior segments compared to the other taxa. For further comparison of Mekong Glyphidrilus species, see Table 2.

We examined allozyme variation in two camaenid tree snails, Amphidromus atricallosus and A. inversus, across two principal regions of Thailand and from Singapore, plus for A. inversus, one site in peninsular Malaysia. Using horizontal starch gel electrophoresis, 13 allozyme loci (11 polymorphic) were screened for A. atricallosus and 18 (5 polymorphic) for A. inversus. Heterozygosity was higher in A. atricallosus (Hexp=0.018-0.201, mean=0.085) than in A. inversus (Hexp=0-0.023, mean= 0.002). Genetic heterogeneity among samples was higher in A. inversus (Fst=0.965) than in A. atricallosus (Fst=0.781). Within A. atricallosus, populations were more differentiated in southern Thailand (Fst=0.551) than in eastern Thailand (Fst=0.144). The high Fst and low Hexp in populations of A. inversus suggest that this species is likely to have experienced a series of strong bottlenecks, perhaps occurring chiefly on offshore continental-shelf islands. The low Fst values of A. atricallosus in eastern Thailand suggest frequent gene flows among populations in this region. The southern and eastern samples of A. atricallosus exhibited fixed allele differences at four loci and great genetic distance (Nei's D=0.485-0.946), suggesting that these two samples may actually represent, or else be evolving into, separate species.

Although landscape features such as mountains and rivers are recognized often as limiting factors to amphibian dispersal and gene flow, a limited number of studies have investigated such patterns across Southeast Asia. A perfect example of this is Thailand, located in one of the world's biodiversity hotspot regions. Thailand represents the corridor between mainland Asia and the Sunda Shelf, a famous and widely recognized biogeographic region, and yet there are few studies on the genetic structure among populations of amphibian species distributed across Thailand. The Southeast Asian tree frog, Chiromantis hansenae has been reported to possess a geographic range that is restricted to Thailand and, presumably, Cambodia. Here, we investigate phylogenetic relationships among C. hansenae populations using partial sequences of the mitochondrial 16S rRNA gene and nuclear POMC gene. Our results reveal two distinct evolutionary lineages within C. hansenae populations in Thailand. The genetic divergence among populations between these two clades is considerable, and results support inter-population divergence, and high genetic differentiation (pairwise FST = 0.97), between two localities sampled in western Thailand (TK1 and TK2), separated from each other by 40 kilometers only. The results suggest that landscape features across Thailand may have a profound impact on patterns of diversification in the country, underscoring the urgent need for fine-scale investigations of genetic structure of endemic and "widespread" species.