The microsnail genera Clostophis and Acinolaemus (Eupulmonata, Pupilloidea, Hypselostomatidae) from central Thailand, with description of three new species

35 The microsnail genera Clostophis and Acinolaemus (Eupulmonata, Pupilloidea, Hypselostomatidae) from central Thailand, with description of three new species Piyoros Tongkerd1, Teerangkul Janjai1, Arthit Pholyotha1, Vukašin Gojšina2, Somsak Panha1,3 , Chirasak Sutcharit1 1 Animal Systematics Research Unit, Department of Biology, Faculty of Science, Chulalongkorn University, Phayathai Road, Patumwan, Bangkok 10330, Thailand 2 Department of Morphology, Systematics and Phylogeny of Animals, University of Belgrade, Faculty of Biology, Studentski trg 16, 11000, Belgrade, Serbia 3 Academy of Science, The Royal Society of Thailand, Bangkok 10300, Thailand Corresponding authors: Somsak Panha ([email protected]); Chirasak Sutcharit ([email protected]; chir[email protected]) Copyright: © Piyoros Tongkerd et al. This is an open access article distributed under terms of the Creative Commons Attribution License (Attribution 4.0 International – CC BY 4.0). Research Article Abstract Hypselostomatid microsnails of the genera Clostophis and Acinolaemus from limestone hills in central Thailand were studied and three new species are described. Clostophis rhynchotes Tongkerd & Panha, sp. nov. is diagnosed by a conical spire, long and descending tuba, 14 spiral striations on the last whorl, and only a single parietal lamella. In addition, a previously known species, C. proboscideus, is redescribed, and variations in its apertural dentition are also documented. In the genus Acinolaemus, two new sympatric species that clearly differ in shell shape are described. Acinolaemus rhamphodontis Tongkerd & Panha, sp. nov. is characterised by a depressed conical spire with a long and descending tuba, and eight apertural dentitions, while A. corusticorus Tongkerd & Panha, sp. nov. possesses a conical shell without a tuba and nine apertural dentitions. Specimens from the type locality of A. ptychochilus (the type species), A. cryptidentatus and A. mueangonensis are re-described and compared with the new species. The living snails of A. mueangonensis and A. rhamphodontis Tongkerd & Panha, sp. nov. possess blackish to translucent bodies. In addition, COI barcoding data for Clostophis and Acinolaemus are provided for the first time. Key words: Chao Phraya River, DNA barcoding, gastropods, limestone, taxonomy Introduction The terrestrial microsnails of Thailand belonging to the family Hypselostomatidae Zilch, 1959 (usually smaller than 5 mm) have been studied for more than 20 years (Panha 1998a, b; Panha and Burch 2005). Currently, this snail group comprises 13 nominal genera, and the species richness in each genus varies from nearly 100 species in Hypselostoma Benson, 1856 to one species in Boysia Pfeiffer, 1849 and two species in Pseudostreptaxis von Möllendorff, 1890 (Schileyko 1998; Páll-Gergely et al. 2019; Jirapatrasilp et al. 2024; Gojšina et al. 2025; MolluscaBase 2025). In the present paper, we focus on the genera Clostophis Benson, 1860 and Acinolaemus Thompson & Upatham, 1997, which are considered to be closely related and comprise morphologically similar species characAcademic editor: Edmund Gittenberger Received: 26 June 2025 Accepted: 12 September 2025 Published: 3 November 2025 ZooBank: https://zoobank.org/ pub:7CE46D4C-F8C1-445B-A8EC20A450AB1036 Citation: Tongkerd P, Janjai T, Pholyotha A, Gojšina V, Panha S, Sutcharit C (2025) The microsnail genera Clostophis and Acinolaemus (Eupulmonata, Pupilloidea, Hypselostomatidae) from central Thailand, with description of three new species. ZooKeys 1258: 35–71. https://doi.org/10.3897/ zookeys.1258.162797 ZooKeys 1258: 35–71 (2025) DOI: 10.3897/zookeys.1258.162797 36 ZooKeys 1258: 35–71 (2025), DOI: 10.3897/zookeys.1258.162797 Piyoros Tongkerd et al.: The microsnail genera Clostophis and Acinolaemus from Thailand terised by small (usually < 2 mm), colourless shells, with or without a detached last whorl (or tuba), and relatively variable apertural dentition (Thompson and Upatham 1997; Páll-Gergely et al. 2020; Páll-Gergely and Hunyadi 2022). The genus Acinolaemus was originally created for eight species and was diagnosed by its colourless shell, its enlarged angular lamella forming a distinctly separated sinulus, and its spiral striation crossed by oblique radial growth lines on the protoconch (Thompson and Upatham 1997; Schileyko 1998). It is believed to be endemic to limestone hills from northern to peninsular Thailand as well as adjactent regions of Myanmar and Malaysia. Subsequently, several species have been added to this genus, so that its distribution range was expanded to include the limestone hills in the Salween Basin in Myanmar, and the Mekong Delta Karsts in Cambodia and Vietnam (Vermeulen et al. 2007, 2019; Changlom et al. 2019; Tongkerd et al. 2024). So far, seven of the 11 currently recognised species of Acinolaemus have been recorded in Thailand (Thompson and Upatham 1997; Changlom et al. 2019; MolluscaBase 2025). The genus Clostophis was originally described as a monotypic genus (Benson 1860) that, for more than 150 years, was assigned to the caenogastropod family Diplommatinidae Pfeiffer, 1856, but since the work of Páll-Gergely et al. (2020) and Páll-Gergely and Hunyadi (2022) is now firmly classified as a speciose genus in the heterobranch family Hypselostomatidae. Currently, the genus comprises 20 nominal species which are mainly distributed in the Indo-Burma region, Peninsular Malaysia, and southern China, while only two species have been recorded in Thailand (Páll-Gergely et al. 2020; Páll-Gergely and Hunyadi 2022). Clostophis proboscideus (Panha & Burch, 2002) has a precise type locality in central Thailand (Panha and Burch 2002, 2005) while C. laidlawi (Collinge, 1902) is only known from the type locality as ‘Jalor’ (= Yala Province, Thailand), although several populations were reported from Malaysia (Collinge 1902; Páll-Gergely and Hunyadi 2022). Chao Phraya River Basin in central Thailand plays a crucial role in the hydrology and economy of the country. The area is characterised by floodplains, highlands, and scattered limestone hills or karsts (Prasongtham and Kanjanapayont 2014; Singtuen and Phajuy 2020). Limestone hills are unique ecosystems that support significant, highly diverse, and unique biota (Clements et al. 2006). For this study, we surveyed the central Thailand limestone karsts for Clostophis and Acinolaemus microsnails and collected numerous specimens from multiple localities. Because this material clearly differed in shell shape, shell sculpture, and apertural dentition from all other known Clostophis and Acinolaemus species, we describe one new Clostophis and two new Acinolaemus species by means of shell morphology and DNA sequence data. In addition, to contribute to a better understanding of their taxonomy, C. proboscideus, A. ptychochilus Thompson & Upatham, 1997 (type species of Acinolaemus), A. cryptidentatus Changlom et al., 2019, and A. mueangonensis Changlom et al., 2019 are re-described based on the specimens collected from their type localities. Materials and methods Specimen sampling The fieldwork was focused on limestone outcrops in the Chao Phraya River Basin in central Thailand. The topography in central Thailand consists of scattered 37 ZooKeys 1258: 35–71 (2025), DOI: 10.3897/zookeys.1258.162797 Piyoros Tongkerd et al.: The microsnail genera Clostophis and Acinolaemus from Thailand small hills or karst towers above alluvial plains. The flat plains in this region are currently impacted by paddy fields and other agricultural uses, and quarrying. Specimens were collected from limestone walls, soil, and litter debris at the base of limestone outcrops, and in rock crevices. Shells were soaked in water with detergent, and dirt or mud was removed manually using fine paintbrushes. Dry shells were examined and imaged with a Leica M205C microscope with a fusion optics and the Leica Application Suite Image System. Additional specimens were photographed by scanning electron microscopy (SEM; JEOL, JSM-6610 LV) for more detailed views of microscopic shell structures. Shell whorls were counted to the nearest quarter whorl (Kerney and Cameron 1979). Shell measurements were taken from digital images by Cell’D Imaging Software (Olympus). Identification of specimens and terminology used to describe the apertural dentition were based on Pilsbry (1918, 1948), Páll-Gergely et al. (2020), Páll-Gergely and Hunyadi (2022), Sutcharit et al. (2023), Gojšina et al. (2025), and Sutcharit et al. (2025). DNA extraction, PCR amplification, and sequencing Total genomic DNA was extracted from tissue samples using the G-spin Genomic DNA Extraction Kit following the manufacturer’s protocol. Approximately 655 bp of the mitochondrial cytochrome c oxidase subunit I (COI) gene was amplified by the polymerase chain reaction (PCR) using the primer pairs LCO1490 and HCO2198 (Folmer et al. 1994). PCR reactions were performed with an initial denaturation step at 94 °C for 3 min, 35 PCR cycles (94 °C for 45 s, 45–46 °C for 45 s, 72 °C for 1 min) and a final extension step at 72 °C for 5 min. Both strands of the amplified products were purified using the DNA-spin Plasmid DNA Purification Kit and were sequenced at Bioneer Corporation, South Korea. Chromatograms were manually corrected for misreads, if necessary, and forward and reverse strands were merged into one sequence file using MEGA v. 7.0 (Kumar et al. 2016). New sequences have been deposited in GenBank (Table 1). Additionally, twelve COI sequences were downloaded from GenBank (Table 1). Ten sequences from six Hypselostoma species were used as part of the ingroup and two sequences from two Pupilla species were used as more distantly related outgroup. Sequence alignment and phylogenetic analysis Sequences were aligned using MEGA v. 7.0 with the default settings (Kumar et al. 2016). COI-sequence divergence was expressed as p-distances, calculated with MEGA v. 7.0 with the ‘pairwise deletion of gaps’ option (Kumar et al. 2016). Bayesian inference (BI) and maximum likelihood (ML) analyses were used to reconstruct phylogenetic relationships through the online CIPRES Science Gateway (Miller et al. 2010). The best-fit model of nucleotide substitution was identified for each sequence partition separately by means of the corrected Bayesian information criterion (BIC) using Kakusan4 (Tanabe 2011). As suggested by the Kakusan4 program, three partitions were designated: the HKY85 model with a gamma distribution for the first codon position of the COI fragment, and the GTR model with a gamma distribution for the second and the third codon positions of the COI fragment. Bayesian inference phylogenies were generated using MrBayes on ACCESS v. 3.2.7a (Ronquist et al. 2012), with two runs for 10 38 ZooKeys 1258: 35–71 (2025), DOI: 10.3897/zookeys.1258.162797 Piyoros Tongkerd et al.: The microsnail genera Clostophis and Acinolaemus from Thailand million generations, sampling every 1000 generations, and the first 25% of obtained trees being discarded as burn-in. Maximum likelihood phylogenetic trees were generated using RAxML-HPC2 on ACCESS v. 8.2.12 (Stamatakis 2014) using a GTRCAT model. Bootstrap support (BS) values were used for assessing branch support in the ML tree with 1000 bootstrap replicates and shown as a percentage. Branch support in the BI tree was evaluated by posterior probabilities (PP). PP values ≥ 0.95 and BS values ≥ 70% were interpreted as positive support for nodes, while lower values were interpreted as weak support. Both ML and BI phylogenetic trees were visualised and modified in FigTree v. 1.4.4 (Rambaut 2018), then visually processed in Adobe Illustrator 2021. Institutional abbreviations CUMZ Chulalongkorn University, Museum of Zoology, Bangkok; NHMUK The Natural History Museum, London; SMF Senckenberg Forschungsinstitut und Naturmuseum, Frankfurt am Main; UF Florida Museum of Natural History, University of Florida. Abbreviations a, angular lamella; b, basal plica; c, columellar lamella; ifpl, infrapalatal plica; ip, infraparietal lamella; itpl, inter palatal plica; lpl, lower palatal plica; p, parietal lamella; pt, palatal tubercle; sbc, subcolumellar lamella; spc, supracolumellar lamella; upl, upper palatal plica. Table 1. Samples used for phylogenetic analysis, with specimen codes, museum registration numbers, GenBank accession numbers, sampling localities, and references. 1 = Hoekstra and Schilthuizen (2011). 2 = Nekola et al. (2015). Taxon Code Museum registration number GenBank accession number (COI) Locality Reference Acinolaemus rhamphodontis sp. nov. HC025-1 Paratype CUMZ 14451 PV698334 Tak, Thailand this study Acinolaemus rhamphodontis sp. nov. HC025-2 Paratype CUMZ 14451 PV698335 Tak, Thailand this study Acinolaemus mueangonensis Changlom et al., 2019 HC026 CUMZ 144554 PV698336 Tak, Thailand this study Acinolaemus colpodon Thompson & Upatham, 1997 HE022-2 CUMZ 14445 PV698337 Chon Buri, Thailand this study Acinolaemus rhamphodon Thompson & Upatham, 1997 HE025-2 CUMZ 14481 PV698338 Chon Buri, Thailand this study Clostophis rhynchotes sp. nov. HC016-2 Paratype CUMZ 14464 PV698339 Nakhon Sawan, Thailand this study Clostophis udayaditinus Sutcharit & Panha, 2025 PUP020 Paratype CUMZ 14467 PV698340 Battambang, Cambodia this study Hypselostoma hungerfordianum von Möllendorff, 1891 - - HM240390 Kelantan, Malaysia 1 Hypselostoma hungerfordianum von Möllendorff, 1891 - - HM240394 Perak, Malaysia 1 Hypselostoma salpinx (van Benthem Jutting, 1961) - - HM240407 Pahang, Malaysia 1 Hypselostoma salpinx (van Benthem Jutting, 1961) - - HM240408 Pahang, Malaysia 1 Hypselostoma frequens (van Benthem Jutting, 1950) - - HM240409 Kelantan, Malaysia 1 Hypselostoma depressispira (van Benthem Jutting, 1949) - - HM240410 Kelantan, Malaysia 1 Hypselostoma serpa (van Benthem Jutting, 1950) (= Paraboysidia tarutao Panha & Burch, 2002) - - HM240411 Kedah, Malaysia 1 Hypselostoma serpa (van Benthem Jutting, 1950) (= Paraboysidia tarutao Panha & Burch, 2002) - - HM240412 Kedah, Malaysia 1 Hypselostoma transitans von Möllendorff, 1894 - - HM240405 Kedah, Malaysia 1 Hypselostoma transitans von Möllendorff, 1894 - - HM240406 Kedah, Malaysia 1 Pupilla loessica Ložek, 1954 - - KM518576 Belyashi, Altai, Russia 2 Pupilla triplicata (Studer, 1820) - - KM518612 Hracholusky, Bohemia, Czech Republic 2 39 ZooKeys 1258: 35–71 (2025), DOI: 10.3897/zookeys.1258.162797 Piyoros Tongkerd et al.: The microsnail genera Clostophis and Acinolaemus from Thailand Results Molecular phylogenetic analyses Our final dataset contained COI sequences from 17 specimens representing 12 taxa in the Hypselostomatidae. Seven species were represented by just a single specimen and five species were represented by two specimens. Additionally, two sequences from two pupillid taxa, Pupilla loessica Ložek, 1954 and P. triplicata (Studer, 1820), were included as outgroup. The final COI alignment had a total length of 655 bp, containing 264 variable sites, 391 invariant sites, 232 parsimony-informative sites, and 32 singleton sites. The BI and ML trees were largely consistent; therefore, only the BI tree is presented in Fig. 1. Both BI and ML analyses supported the monophyly of the family Hypselostomatidae (BB/PP: 1/100). However, the phylogenetic relationships within this family remain unresolved. Although six species of Hypselostoma formed a monophyletic group in the BI analysis with weak nodal support, they Figure 1. Bayesian inference tree from analysis of the mitochondrial cytochrome c oxidase subunit I (COI) sequence data of species in Clostophis, Acinolaemus, and Hypselostoma. Numbers on branches indicate the Bayesian posterior probabilities (PP) and ML bootstrap (BS) support values. Blue colour represents Acinolaemus rhamphodontis sp. nov. clade. Yellow colour represents Clostophis rhynchotes sp. nov. clade. Representative shells of Clostophis and Acinolaemus are shown but not to scale. 40 ZooKeys 1258: 35–71 (2025), DOI: 10.3897/zookeys.1258.162797 Piyoros Tongkerd et al.: The microsnail genera Clostophis and Acinolaemus from Thailand were not retrieved as a monophyletic group in the ML tree. Similarly, the monophyly of the genera Clostophis and Acinolaemus was not supported. Conversely, the sister group relationship of A. rhamphodontis sp. nov. and A. mueangonensis was well-supported (BB/PP: 1/100). In contrast, the remaining Acinolaemus species (A. rhamphodon and A. colpodon) were grouped together with two Clostophis species (C. rhynchotes sp. nov. and C. udayaditinus) in a separate clade (BB/PP: 0.87/84). This phylogenetic pattern suggests that current generic boundaries between Clostophis and Acinolaemus may not accurately reflect their evolutionary relationships and highlights the need for further taxonomic revision within the group. The mean interspecific p-distance among the four Acinolaemus species varied from 14.4% to 18.4% and no intraspecific sequence divergence within our examined specimens of A. rhamphodontis sp. nov. (Table 2). The p-distance between C. rhynchotes sp. nov. and C. udayaditinus was 15.6% (Table 2). The average interspecific p-distance for the six Hypselostoma species varied from 13.7% to 21.9% and the average intraspecific sequence divergence within Hypselostoma varied from 0% to 6.7% (Table 2). Systematics Family Hypselostomatidae Zilch, 1959 Clostophis Benson, 1860 Clostophis Benson, 1860: 95. Kobelt 1902: 484. Thiele 1929: 111. Páll-Gergely et al. 2020: 351, 352. Páll-Gergely and Hunyadi 2022: 419. Preece et al. 2022: 145, 256. Montapiculus Panha & Burch, 2002: 148. Type species: Montapiculus proboscidea Panha & Burch, 2002. Panha and Burch 2005: 38, 109. Type species. Clostophis sankeyi Benson, 1860, by monotypy. Remarks. The genus was recently revised by Páll-Gergely et al. (2020) and Páll-Gergely and Hunyadi (2022), who recorded two species from southeastern Myanmar, six species from Laos, six species from Vietnam, two species from Peninsular Malaysia, and two species from Thailand. All these species share Table 2. Intraand interspecific COI p-distances among Acinolaemus, Clostophis, and Hypselostoma. Taxa 1 2 3 4 5 6 7 8 9 10 11 12 1. A. rhamphodontis sp. nov. 0 2. A. rhamphodon 0.184 - 3. A. colpodon 0.167 0.144 - 4. A. mueangonensis 0.155 0.183 0.179 - 5. C. rhynchotes sp. nov. 0.190 0.163 0.134 0.175 - 6. C. udayaditinus 0.179 0.131 0.131 0.184 0.156 - 7. H. transitans 0.210 0.187 0.182 0.196 0.192 0.197 0.031 8. H. serpa 0.174 0.172 0.164 0.190 0.163 0.173 0.137 0.003 9. H. salpinx 0.204 0.192 0.168 0.209 0.185 0.197 0.167 0.147 0 10. H. hungerfordianum 0.182 0.176 0.154 0.179 0.170 0.181 0.139 0.137 0.151 0.067 11. H. frequens 0.219 0.218 0.192 0.212 0.202 0.214 0.179 0.166 0.169 0.147 - 12. H. depressispira 0.198 0.198 0.183 0.195 0.177 0.197 0.161 0.166 0.145 0.139 0.179 - 41 ZooKeys 1258: 35–71 (2025), DOI: 10.3897/zookeys.1258.162797 Piyoros Tongkerd et al.: The microsnail genera Clostophis and Acinolaemus from Thailand strong spiral ridges, usually with one or many apertural dentitions (except for four species with no dentition), and a more or less detached last whorl (tuba). Other characters, such as shell shape and tuba length, show large intraspecific variation (although they can be useful in some cases to distinguish species). Based on its wide distribution and its substantial morphological variability, the genus can be divided into three phenotypic species groups: I. Clostophis sankeyi species group: with 0–4 apertural dentitions, a long (~1/4 whorl or more) and descending tuba, and a conical shell shape with strongly concave sides. This group consists of five species (including Clostophis rhynchotes sp. nov.) distributed in Myanmar, Thailand, and northern Vietnam (Fig. 2A, Table 3). II. Clostophis charybdis species group: with 0–2 apertural dentitions, no to short tuba, moderately to strongly conical shell, concave sides, and aperture opened laterally to sublaterally. This species group comprises ten species, of which nine species are distributed mainly in the central to northern Annamite Ranges in Laos and central to northern Vietnam; one species is found in southern Myanmar (Fig. 2A, Table 3). III. Clostophis bactrianus species group: with four or five strong apertural dentitions, absent to short tuba, and conical to slightly concave sides. This group consists of five species with disjunct distributions. Three speFigure 2. Approximate type localities. A. All Clostophis species. Colours indicate different species groups; numbers correspond to the species listed in Table 3; B. All Acinolaemus species; numbers correspond to the species listed in Table 4. 42 ZooKeys 1258: 35–71 (2025), DOI: 10.3897/zookeys.1258.162797 Piyoros Tongkerd et al.: The microsnail genera Clostophis and Acinolaemus from Thailand Table 3. Species list, diagnostic characters, and type localities of Clostophis species. Numbers in species column refer to those in Figure 2A. The superscript numbers indicate references: 1 = Collinge (1902); 2 = van Benthem Jutting (1961); 3 = Panha and Burch (2002); 4 = Páll-Gergely et al. (2015); 5 = Páll-Gergely et al. (2017); 6 = Páll-Gergely et al. (2020); 7 = Páll-Gergely and Hunyadi (2022); 8 = Sutcharit et al. (2025); 9 = this study. n/a = data not available. Species Shell shape Tubular Protoconch Aperture opening Apertural dentition Type locality C. sankeyi species group 1 C. sankeyi Benson, 18606concave-conical long descending pitted and spirally striated ventral to subventral 1 (parietal) or 2 (parietal, palatal) Myanmar, Mon State 2 C. proboscideus (Panha & Burch, 2002)3,6,9 concave-conical long and descending pitted and spirally striated ventral to subventral 2 (parietal, palatal) Thailand, Nakornsawan 3 C. rhynchotes sp. nov.9concave-conical long and descending pitted and spirally striated ventral to subventral 1 parietal Uthai Thani, Thailand 4 C. udayaditinus Sutcharit & Panha, 20258 concave-conical long and descending pitted and spirally striated ventral to subventral 4 (parietal, infraparietal, palatal, columellar) Cambodia, Battambang 5 C. yoga Páll-Gergely & Hunyadi, 20227 concave-conical long and descending spiral striations subventral none Vietnam, Thanh Hoa C. charybdis species group 6 C. charybdis Páll-Gergely & Hunyadi, 20227 concave-conical very short to absent pitted and spirally striated lateral 2 (parietal, palatal) Vietnam, Lang Son 7 C. candidus Páll-Gergely & Hunyadi, 20227 conical, slightly concave side short spirally striated sublateral none Vietnam, Lang Son 8 C. incurvus Páll-Gergely & Vermeulen, 20206 conical, straight to slightly convex side very short to absent n/a lateral 2 (parietal, palatal) Vietnam, Quang Ninh 9 C. infantilis Páll-Gergely, 20207conical, straight side absent pitted and spirally striated lateral none Vietnam, Haiphong 10 C. koilobasis Páll-Gergely & Vermeulen, 20206 low conical, concave side very short to absent spirally striated lateral 1 parietal Laos, Khammouan 11 C. multiformis Páll-Gergely & Reischütz, 20206 conical, straight side short to slightly long pitted and spirally striated lateral or sublateral 1 parietal Laos, Khammouane 12 C. neglectus (van Benthem Jutting, 1961)2,6 conical, straight side very short to absent spirally striated lateral 2 (parietal, palatal) Malaysia, Pahang 13 C. obtusus Páll-Gergely & Grego, 20206 conical, straight to slightly convex side very short to absent pitted and spirally striated lateral none Laos, Khammouane 14 C. platytrochus Páll-Gergely & Hunyadi, 20206 conical, slightly concave side short and descending spirally striated sub lateral 2 (parietal, palatal) Vietnam, Da Nang 15 C. stochi (Páll-Gergely & Jochum, 2017)5 conical, slightly convex side very short to absent spirally striated lateral 2 (parietal, palatal) Vietnam, Ha Giang 16 C. thinbowguensis Páll-Gergely & Hunyadi, 20227 conical, straight side short n/a sublateral 1 parietal Myanmar, Tanintharyi C. bactrianus species group 17 C. bactrianus Páll-Gergely & Hunyadi, 20227 conical, straight side short spirally striated lateral 4 (parietal, palatal, basal, columellar) Malaysia, Pahang 18 C. lacrima (Páll-Gergely & Hunyadi, 2015)4 concave-conical very short to absent pitted and spirally striated lateral 4 (parietal, palatal, basal, columellar) China, Guangxi 19 C. laidlawi (Collinge, 1902)1,7 conical, slightly concave side very short to long and descending spirally striated lateral or sublateral 5 (parietal, angular, palatal, basal, columellar) Thailand, Yala 20 C. obliquus Páll-Gergely & Hunyadi, 20227 conical, slightly concave side very short to absent pitted and spirally striated lateral 4 (parietal, palatal, basal, columellar) China, Guangxi 21 C. socialis (Páll-Gergely & Hunyadi, 2015)4 concave-conical short pitted and spirally striated lateral 4 (parietal, palatal, basal, columellar) China, Guangxi cies from southern China tend to have long denticles deeper inside the aperture, whereas the two species from Peninsular Malaysia tend to have short denticles situated near the apertural lip (Fig. 2A, Table 3). 43 ZooKeys 1258: 35–71 (2025), DOI: 10.3897/zookeys.1258.162797 Piyoros Tongkerd et al.: The microsnail genera Clostophis and Acinolaemus from Thailand Clostophis proboscideus (Panha & Burch, 2002) Figs 3–5, Table 3 Montapiculus proboscidea Panha & Burch, 2002 [1999]: 148, figs 4–7. Type locality: Teppratan mountain, Nakornsawan Province. Panha and Burch 2005: 109, fig. 94. Clostophis proboscideus—Páll-Gergely et al. 2020: 364, figs 1j, 3b. Material examined. Holotype. Thailand • Wat Khao Huai Lung (former Teppratan mountain), Ban Daen, Banphot Phisai District, Nakhon Sawan Province; 15°55'35.4"N, 99°52'23.2"E; S. Panha leg.; CUMZ 14458 (former Ver-079). Paratypes. Thailand • 2 shells (Fig. 3A–C); same data as for holotype; CUMZ 14457 (former Ver-080). Other material. Thailand • 2 shells (Fig. 4A–C); Wat Khao Huai Lung (former Teppratan mountain), Ban Daen, Banphot Phisai District, Nakhon Sawan Province; 15°55'35.4"N, 99°52'23.2"E; P. Tongkerd leg.; CUMZ 14456. • 2 shells + 2 juveniles; Wat Tham Bo Ya, Nong Krot, Mueang Nakhon Sawan District, Nakhon Sawan Province; 15°43'49.7"N, 99°56'45.7"E; P. Tongkerd leg.; CUMZ 14465. • 4 shells + 1 juvenile (Fig. 5A–D); Khao Pathawee, Taluk Du, Thap Than District, Uthai Thani Province; 15°28'25.2"N, 99°45'30.3"E; P. Tongkerd leg.; CUMZ 14429. • 105 shells + 8 juveniles; same collection data as preceding; CUMZ 14463. • 4 shells; Phu Toei, Huai Khamin, Dan Chang District, Suphan Buri Province; S. Panha leg.; CUMZ 15351. Diagnosis. Shell concave-conical and with long and descending tuba. Apertural dentition with one parietal lamella and one weak palatal plica. Umbilicus wide. Description. Shell concave-conical, colourless; growing regularly; last whorl slightly expanded. Shell height 1.7–1.9 mm (including tuba) and shell width 1.6–1.7 mm. Apex large and rounded; protoconch ~1½ whorls, pitted and with very narrow spiral striae. Whorls ~4½–5 (excluding tuba), rounded and convex; suture wide and deep. Shell surface with strong, elevated and rather equidistant spiral striae (18–22 on body whorl in frontal view), and crossed with weak to strong and irregular radial growth lines. Sometimes growth lines on earlier whorls strong, thus making reticulated surface. Penultimate whorl seemingly sunken into last whorl. Last whorl slightly shouldered to rounded; tuba long, ~¼ whorl, strongly descending, curving and slightly twisted. Aperture subovate, open ventrally to subventrally; peristome thin, slightly expanded and with weak depression on parietal side. Apertural dentitions two: parietal lamella relatively strong, high, and situated near apertural lip; palatal plica moderate, low, and situated slightly deeper inside aperture (infrapalatal plica occasionally present). Umbilicus widely opened, occupies ~½ of shell width and showing all preceding whorls. Differential diagnosis. Clostophis proboscideus differs from C. sankeyi and C. yoga Páll-Gergely & Hunyadi, 2022 by having more numerous and more tightly coiled whorls, lower spire, and wider umbilicus. Meanwhile, C. sankeyi has weaker spiral striations, and C. yoga possesses no apertural dentition, has a narrower umbilicus (< 1/3 of shell width), and with spiral striations throughout the protoconch (Benson 1860; Páll-Gergely and Hunyadi 2022; Preece et al. 2022). Clostophis proboscideus differs from the long and descending tuba morphs of C. multiformis Páll-Gergely & Reischütz, 2020 and C. laidlawi by having a con- 50 ZooKeys 1258: 35–71 (2025), DOI: 10.3897/zookeys.1258.162797 Piyoros Tongkerd et al.: The microsnail genera Clostophis and Acinolaemus from Thailand Remarks. The genus currently contains 11 species (MolluscaBase 2025) with few to many apertural dentitions, a shell with ascending to descending tuba or without tuba, and a shell surface with rectangular reticulations to prominent spiral striations (Table 4; Thompson and Upatham 1997). These variations are possibly the cause that rendered the generic boundary ambiguous and necessitates intensive systematic revision. Acinolaemus ptychochilus Thompson & Upatham, 1997 Figs 8, 9, Table 4 Acinolaemus ptychochilus Thompson & Upatham, 1997: 225, 226, figs 7–11. Type locality: Ban Prang Ma-O, Doi Pha San Sao (Mountain), Chiang Mae [= Chiang Mai] Province, Thailand. Acinolaemus ptychochilus—Schileyko 1998: 255, fig. 316. Type material examined. Holotype. Thailand • height 1.5 mm, width 1.3 mm (Fig. 8); Doi Pha San Sao, Ban Prang Ma-O, Chiang Mai Province; F.G. Thompson leg.; UF 113502. Other material. Thailand • 4 shells (Fig. 9); limestone near Chai Prakan Highway Division (~43 km to Chieng Dao District), Chai Prakan District, Chiang Mai Province; S. Panha leg.; CUMZ 15360. Diagnosis. Shell concave-conical, peristome weakly expanded. Apertural dentitions eleven or more, long and reaching peristome edge in form of prominent knobs: three on parietal wall, palatal tubercle, four on palatal wall, one basal, and two on columellar wall. Re-description. Shell conical, with concave sides, colourless; spire low to high and growing regularly; last whorl expanded. Shell height 1.3–1.4 mm and shell width 1.4–1.5 mm. Apex blunt; protoconch ~1½–2 whorls, with narrow and fine spiral striae. Whorls ~4–5, rounded and convex; suture wide, well impressed, and deep. Shell surface sculptured with equidistantly spaced spiral striae, crossed with irregular radial growth lines making a rectangular reticulated sculpture throughout shell. Penultimate whorl regularly coiled; last whorl rounded. Aperture subovate; peristome thickened, slightly expanded and with weak depression on parietal side. Aperture with approx. ten or eleven dentitions and with strong knobs reaching peristome edge. Parietal wall with three lamellae: parietal lamella large, outer part low, inner part long with tall ridge and located deeper inside aperture; infraparietal lamella long and low; angular lamella long, consisting of two peaks of prominent tall ridges which are interrupted by a low wide ridge. Palatal tubercle rectangular, situated at peristome edge and continuous with upper palatal plica. Three tiny low plicae present (not reaching peristome edge) in sinulus. Palatal wall with four plicae: upper-, inter-, lower-, and infra-palatal plicae connecting peristome edge with prominent tubercles, then continuing with narrow, low ridge, and becoming a stronger fold deep inside aperture. Folding of interand lower-palatal plicae larger than upperand infra-palatal plicae. Infrapalatal plica hooked (in holotype). Basal plica a low ridge and similar to infrapalatal plica. Columellar wall with three or four lamellae: columellar lamella strong, distinct, and tall ridge; supracolumellar and one or two subcolumellar lamellae present as low ridges. Umbilicus widely perforated, ~⅓ of shell width, rounded and deep. 51 ZooKeys 1258: 35–71 (2025), DOI: 10.3897/zookeys.1258.162797 Piyoros Tongkerd et al.: The microsnail genera Clostophis and Acinolaemus from Thailand Figure 8. Acinolaemus ptychochilus from Chiang Mai Province. A. Holotype UF 113502 with enlarged aperture from three different angles; B. Inset of hooked infrapalatal plica; C. Protoconch and earlier whorl sculptures. An asterisk (*) indicates small lamella and plica situated in posterior chamber. Photo credit: B. Páll-Gergely. Distribution. Acinolaemus ptychochilus is known from the type locality in Chiang Dao District, Chiang Mai Province in northern Thailand (Thompson and Upatham 1997). Remarks. The specimens from Chai Prakan District, which is ca 30 km north of the type locality, have only faint protoconch and teleoconch sculptures due to shell weathering. However, the original sculpture remains near the suture and around the umbilical area. These specimens differ slightly from the holotype (Fig. 8A) in having a slightly more depressed shell, a less expanded last 52 ZooKeys 1258: 35–71 (2025), DOI: 10.3897/zookeys.1258.162797 Piyoros Tongkerd et al.: The microsnail genera Clostophis and Acinolaemus from Thailand Figure 9. Acinolaemus ptychochilus from Chiang Mai Province. A. Specimen CUMZ 15360 with enlarged aperture from three different angles; B. Protoconch and earlier whorl sculptures. An asterisk (*) indicates small lamella and plica situated in posterior chamber. whorl, infrapalatal plica that is not hooked (hooked in the type), and two subcolumellar lamellae (one in the type); however, the remaining apertural dentitions are identical to those in the holotype. Acinolaemus cryptidentatus Changlom, Chan-ard & Dumrongrojwattana, 2019 Fig. 10, Table 4 Acinolaemus cryptidentatus Changlom, Chan-ard & Dumrongrojwattana, 2019: 158, 159, fig. 2. Type locality: Tham Wua (Wua Cave), Mueang District, Mae Hong Son Province, Thailand. Tongkerd et al. 2024: 164, figs 1b, 2b. 53 ZooKeys 1258: 35–71 (2025), DOI: 10.3897/zookeys.1258.162797 Piyoros Tongkerd et al.: The microsnail genera Clostophis and Acinolaemus from Thailand Material examined. Thailand • 3 shells (Fig. 10A, B); Mae La Na Cave, Pang Mapha District, Mae Hong Son Province; 19°34'30.8"N, 98°12'56.3"E; S. Panha leg.; CUMZ 15362. Diagnosis. Shell concave-conical, peristome weakly expanded, rectangular reticulated sculpture present on shell surface. Apertural dentitions nine: three on parietal wall, three on palatal wall, one basal, and two on columellar wall. Palatal tubercle prominent and located between upperand inter-palatal plicae. Re-description. Shell weakly concave-conical, colourless; spire high and growing regularly; last whorl expanded. Shell height 1.6–1.8 mm and shell width 1.5–1.6 mm. Apex blunt; protoconch ~2 whorls with conspicuous spiral striae. Whorls ~4–5, bluntly shouldered, and convex; suture wide, well impressed, and deep. Shell surface sculptured with equidistantly spaced spiral striae, crossed with discontinuous and irregular radial ridge-like growth lines making rectangular reticulated sculpture throughout shell. Penultimate whorl regularly coiled to slightly sunken into last whorl. Last whorl rounded to weakly shouldered and Table 4. Species list, diagnostic characters, and type localities of Acinolaemus species. Numbers in species column refer to those in Fig. 2B. The superscript numbers indicate references: 1 = Thompson and Upatham (1997); 2 = Schileyko (1998); 3 = Vermeulen et al. (2007); 4 = Vermeulen et al. (2019); 5 = Changlom et al. (2019); 6 = Tongkerd et al. (2024); 7 = this study. Species Shell shape Shell sculpture Number of dentitions on Palatal tubercle Type locality parietal wall palatal wall columellar wall 1. A. cryptidentatus Changlom et al., 20195,6,7 conical, slightly concave side, no tuba rectangular reticulation 3 (parietal, angular, infraparietal) 4 (3 palatals, basal) 1 columella present Thailand, Mae Hong Son 2. A. dayanum (Stoliczka, 1871)6 concave-conical, no tuba rectangular reticulation 3 (parietal, angular, infraparietal) 4 (3 palatals, basal) 1 columella present Myanmar, Mon 3. A. mueangonensis Changlom et al., 20195,7 conical, straight to concave side, no tuba rectangular reticulation 3 (parietal, angular, infraparietal) 4 (3 palatals, basal) 1 or 2 columellae present Thailand, Chiang Mai 4. A. ptychochilus Thompson & Upatham, 19971,2,7 conical, slightly concave side, no tuba rectangular reticulation 3 (parietal, angular, infraparietal) 5 (4 palatals, basal) 3 (columellae) present Thailand, Chiang Mai 5. A. corusticorus, sp. nov.7 conical, slightly concave side, no tuba rectangular reticulation 3 (parietal, angular, infraparietal) 3 or 4 palatals 2 (columellae) present Thailand, Tak 6. A. rhamphodontis sp. nov.7 depressed conical, concave side, long and descending tuba rectangular reticulation 3 (parietal, angular, infraparietal) 3 (2 palatals, basal) 2 (columellae) present Thailand, Tak 7. A. carcharodon Vermeulen et al., 20074 depressed conical, curved side, short descending tuba prominent spiral striations 2 (parietal, angular) 2 (parietal, basal) absent absent Vietnam, Kien Giang 8. A. pyramidalis (Vermeulen et al., 2007)3 conical, slightly concave side, short and descending tuba prominent spiral striations 2 (parietal, angular) 2 palatals 1 columella or absent absent Vietnam, Kien Giang 9. A. rectus Vermeulen et al., 20193 conical, slightly concave side, short and ascending tuba prominent spiral striations 2 (parietal, angular) 2 palatals 1 columella absent Cambodia, Kampot 10. A. rhamphodon Thompson & Upatham, 19971 concave-conical, no tuba prominent spiral striations 2 (parietal, angular) 2 palatals 1 columella weak Thailand, Chachoengsao 11. A. stenopus Thompson & Upatham, 19971 conical, slightly concave side, no tuba prominent spiral striations 2 (parietal, angular) 2 palatals 1 columella present Thailand, Chanthaburi 12. A. colpodon Thompson & Upatham, 19971 conical, slightly concave side, long and ascending tuba prominent spiral striations 1 angular 1 palatal 1 columella weak Thailand, Rayong 13. A. sphinctinion Thompson & Upatham, 19971 conical, straight side, very short and ascending tuba prominent spiral striations 2 (angulo-parietal, infraparietal) 2 palatals 1 columella absent Thailand, Pra Chuap Khiri Khan 54 ZooKeys 1258: 35–71 (2025), DOI: 10.3897/zookeys.1258.162797 Piyoros Tongkerd et al.: The microsnail genera Clostophis and Acinolaemus from Thailand Figure 10. Acinolaemus cryptidentatus from Mae Hong Son Province. A. Specimen CUMZ 15362 with enlarged aperture from three different angles; B. Protoconch and earlier whorl sculptures. An asterisk (*) indicates small lamella and plica situated in posterior chamber. flattened below periphery. Aperture subovate; peristome weakly expanded. Aperture with eight dentitions. Parietal wall with three lamellae: parietal lamella large, outer part low and reaching peristome edge, inner part strongly developed with long tall ridge located deeper inside aperture; infraparietal lamella prominent with a high ridge; angular lamella slightly curved, reaching peristome edge, a strong, tall ridge, long deeper inside aperture and with deep incision medially. Palatal tubercle strong, triangular, and situated on peristome edge between upperand inter-palatal plicae. Three tiny and low plicae present in 55 ZooKeys 1258: 35–71 (2025), DOI: 10.3897/zookeys.1258.162797 Piyoros Tongkerd et al.: The microsnail genera Clostophis and Acinolaemus from Thailand sinulus. Palatal wall with three plicae: upper-, inter-, and lower-palatal plicae with tall ridges and situated slightly inside aperture. Lower palatal plica tall and more prominent than upperand inter-palatal plicae. Basal plica prominent with tall ridge. Columellar wall with two lamellae: columellar lamella is a very tall and distinct ridge; subcolumellar lamella a strong ridge and almost same size as basal plica. Umbilicus widely perforated, ~⅓ of shell width, rounded and deep. Distribution. Acinolaemus cryptidentatus is known from the type locality at Tham Wua Cave, Mae Hong Son Province in northern Thailand (Changlom et al. 2019), and subsequently reported from Shan State, Myanmar (Tongkerd et al. 2024). Remarks. The specimens from Mae La Na Cave in Pang Mapha District, which is ca 15 km east of the type locality are almost identical to the type specimen in shell shape, sculpture, and most of the apertural dentitions. However, these specimens have a prominent subcolumellar lamella, which was probably overlooked in the original description. Acinolaemus mueangonensis Changlom, Chan-ard & Dumrongrojwattana, 2019 Figs 11–13B, C, Table 4 Acinolaemus mueangonensis Changlom, Chan-ard & Dumrongrojwattana, 2019: 159–161, fig. 3. Type locality: Tham Mueang On [Mueang On Cave], Mae On District, Chiang Mai Province, Thailand. Acinolaemus muangonensis [sic]—Changlom et al. 2019: 155 (abstract), 160 (figure caption). Material examined. Thailand • 8 shells (Fig. 11A, B); Mueang On Cave, Ban Sa Ha Khon, Mae On District, Chiang Mai Province; 18°47'13.2"N, 99°14'16.6"E; S. Panha leg.; CUMZ 15361 [type locality]. • 53 shells (Figs 12A, 13B, C); Phra Phutthabat Doi Khao Nam, Ban Na Subdistrict, Sam Ngao District, Tak Province; 17°14'54.7"N, 98°56'17.1"E; P. Tongkerd leg.; CUMZ 14455.1. • 1 shell (Fig. 12B); same data as preceding; CUMZ 14455.3. • 10 specimens in ethanol (Fig. 12C, D); same data as preceding; CUMZ 14455.4. Diagnosis. Shell concave-conical, brownish and with rectangular reticulated sculpture. Apertural dentitions nine, long and reaching peristome edge: three on parietal wall, three on palatal wall, one basal and one on columella wall. Palatal tubercle continuous with upper palatal plica. Re-description. Shell conical, concave-sided, brownish to pale brown; spire low to high, growing regularly, and sometimes slightly bent; last whorl expanded. Shell height 1.8–1.9 mm and shell width 1.7–1.8 mm. Apex blunt; protoconch spirally striated, ~2. Whorls ~5–6, rounded and convex; suture wide and well impressed. Shell surface sculptured with equidistantly spaced spiral striae, crossed with discontinuous and irregular radial growth lines making rectangular reticulated sculpture throughout shell. Penultimate whorl regularly coiled or sunken into last whorl. Last whorl bluntly shouldered and flattened below periphery. Aperture subcircular; peristome thickened and slightly expanded. Aperture with eight dentitions and with strong knobs reaching peristome edge. Parietal wall with three lamellae: parietal lamella large, outer part low, and inner part strongly developed and high, long deeper inside aperture; infraparietal 56 ZooKeys 1258: 35–71 (2025), DOI: 10.3897/zookeys.1258.162797 Piyoros Tongkerd et al.: The microsnail genera Clostophis and Acinolaemus from Thailand Figure 11. Acinolaemus mueangonensis from the type locality in Chiang Mai Province. A. Specimen CUMZ 15361 with enlarged aperture from three different angles; B. Protoconch and earlier whorl sculptures. An asterisk (*) indicates small lamella and plica situated in posterior chamber. lamella long and low; angular lamella prominent, high, long deeper inside aperture, and with slightly narrow and low ridge in middle. Palatal tubercle triangularly shaped, situated on peristome edge and connected to upper palatal plica. One or two tiny and low plicae (not reaching peristome edge) may be present in sinulus. Palatal wall with three plicae: upper-, inter-, and lower-palatal plicae ending on peristome edge as prominent tubercles, then continuing as narrow and low, but becoming strong deeper inside aperture. Basal plica low and long 57 ZooKeys 1258: 35–71 (2025), DOI: 10.3897/zookeys.1258.162797 Piyoros Tongkerd et al.: The microsnail genera Clostophis and Acinolaemus from Thailand Figure 12. Acinolaemus mueangonensis from Tak Province. A. Specimen CUMZ 14455.1 under light microscope showing brownish shell and plicae; B. Specimen CUMZ 14455.3 with enlarged aperture from two different angles; C, D. Living snails CUMZ 14455.4: C. Snail crawling on Hypselostoma pendulum; D. Crawling on wet paper towel (shell width ~1.8 mm). An asterisk (*) indicates small lamella and plica situated in posterior chamber. deeper inside aperture. Columellar lamella strong, high, and continuing deep inside aperture. Umbilicus widely perforate, ~⅓ of shell width, rounded and deep. Living animal. Snails are typically stylommatophoran with two pairs of tentacles. Upper tentacles are long, stout, cylindrical tubes, dark greyish, and with dark eye spots on the tip (Fig. 12C, D). Lower pairs are very short to knobshaped and can be seen clearly in full extension in moving snails. Animal with 58 ZooKeys 1258: 35–71 (2025), DOI: 10.3897/zookeys.1258.162797 Piyoros Tongkerd et al.: The microsnail genera Clostophis and Acinolaemus from Thailand Figure 13. A. Synoptic photo of sympatric Acinolaemus species, specimens sorted from soil sample collected at Tak Province; B, C. Acinolaemus mueangonensis, specimen CUMZ 14455.1 with brownish shell in lateral and apertural views showing long plicae visible throughout the translucent shell; D, E. Acinolaemus corusticorus sp. nov., paratype CUMZ 14455.2 with colourless shell in lateral and apertural views showing short plicae visible throughout the translucent shell. short body, anterior-dorsal side with grey to blackish pigmentation, and posterior body and foot pale greyish to semi-translucent. The snails tend to cover their shell with soil, mud, or dirt. Distribution. Acinolaemus mueangonensis has a distribution beyond its type locality in northern Thailand (Changlom et al. 2019), since during this study several specimens were collected in soil samples taken at the base of limestone cliffs and outcrops in Tak Province, ca 170 km south of the type locality. 59 ZooKeys 1258: 35–71 (2025), DOI: 10.3897/zookeys.1258.162797 Piyoros Tongkerd et al.: The microsnail genera Clostophis and Acinolaemus from Thailand Remarks. The specimens from the type locality examined herein have shell sculpture and apertural dentition similar to the original description and the type specimens (Changlom et al. 2019), except for the presence of a tiny plica in the sinulus, which was not mentioned in the original description. The specimens from central Thailand (Tak population) show the same distinguishing characters as the type specimens: a brownish shell, rectangular reticulated sculpture, and long denticles deep inside the aperture and becoming strong knobs when reaching the peristome edge. In addition, the parietal lamella has a low ridge near the peristome that becomes a tall ridge inside the aperture, and the angular lamella has a high ridge with a narrow and low ridged incision in the middle. However, the central Thailand population (Fig. 12A, B) differed from typical specimens (Fig. 11A, B) in having a more concave-sided shell, a more elevated and slender spire (height 2.1–2.2 mm and width 1.7–1.8 mm), and the penultimate whorl slightly sunken into the last whorl. Additionally, the central Thailand population has a basal plica instead of a subcolumellar lamella, while a typical shell has a subcolumellar lamella instead of a basal plica. However, the distinction between the subcolumellar lamella and the basal plica is sometimes difficult and not a reliable character for species distinction and is subject to intraspecific variability. Therefore, we provisionally recognise the central population as conspecific with A. mueangonensis, since no concrete difference in morphology could be observed. DNA sequence data are needed to resolve this ambiguity. Acinolaemus corusticorus Tongkerd & Panha, sp. nov. https://zoobank.org/78BD0129-3EAB-4593-AA77-8FCED5DCD0DA Figs 13D, E, 14, 15, Table 4 Type material. Holotype. Thailand • height 2.2 mm, width 1.9 mm (Fig. 14A, B); Phra Phutthabat Doi Khao Nam, Ban Na, Sam Ngao District, Tak Province; 17°14'54.7"N, 98°56'17.1"E; P. Tongkerd leg.; CUMZ 15363.1. Paratypes. Thailand • 2 shells (Fig. 15A, B); same data as for holotype; CUMZ 15363.2. • 45 shells (Fig. 13D, E); same data as for holotype; CUMZ 14455.2. • 2 shells; same data as for holotype; NHMUK 20250360. • 2 shells; same data as for holotype; SMF. Diagnosis. Shell concave-conical, colourless, and with rectangular reticulations. Nine apertural dentitions, reaching peristome edge: three on parietal wall, palatal tubercle, four on palatal wall, and two on columellar wall. Palatal tubercle continuous with upper palatal plica. Description. Shell concave-conical, colourless; spire high and growing regularly; last whorl expanded. Shell height 2.1–2.3 mm and shell width 1.8– 1.9 mm. Apex blunt; protoconch ~2 whorls with fine spiral striae. Whorls 4–5, rounded and convex; suture wide and well impressed. Shell surface sculptured with equidistantly spaced spiral striae, crossed with irregular radial growth lines making a rectangular reticulated sculpture throughout shell. Penultimate whorl regularly coiled; last whorl bluntly shouldered and flattened below periphery. Aperture subovate; peristome thickened and slightly expanded. Aperture with nine or ten dentitions and with more or less strong knobs reaching peristome edge. Parietal wall with three lamellae: parietal lamella large, strongly 66 ZooKeys 1258: 35–71 (2025), DOI: 10.3897/zookeys.1258.162797 Piyoros Tongkerd et al.: The microsnail genera Clostophis and Acinolaemus from Thailand A. carcharodon has a sunken spire and four dentitions (parietal, hooked angular, palatal, and basal), A. pyramidalis has a conical spire and four to five dentitions (parietal, angular, two palatals, and a very inconspicuous columellar), and A. rectus has a conical spire, last whorl rounded, tuba slightly ascending and five dentitions (parietal, angular, two palatals, and columellar) (Vermeulen et al. 2007, 2019). This new species has a long descending tuba similar to several species in the Clostophis sankeyi species group. It differs by having rectangular reticulated shell sculpture, with eight apertural dentitions (parietal, infra parietal, angular, two palatals, basal, and two columellar) reaching the peristome edge, and with a palatal tubercle present. Furthermore, A. rhamphodontis sp. nov. has 14–16 dash-like spiral striae on the last whorl and the penultimate whorl not sunken, whereas C. sankeyi, C. proboscideus, C. yoga, and C. udayaditinus have 18–20 or more continuous spiral striae on the last whorl, and the penultimate whorl sunken into the last whorl. Clostophis yoga also has a narrower umbilicus, <1/3 of the shell width, than the new species (Benson 1860; Páll-Gergely et al. 2020; Páll-Gergely and Hunyadi 2022; Sutcharit et al. 2025). Finally, A. rhamphodontis sp. nov. also differs from C. rhynchotes sp. nov. by having a rectangular reticulated shell surface and dash-like spiral striae on the last whorl, while C. rhynchotes sp. nov. has a weak reticulated shell surface and continuous spiral striae on the last whorl. Etymology. The specific name rhamphodontis is from two Greek words: rhamphos meaning ‘curving beak’ and odontos meaning ‘tooth’. Together they refer to the tuba that curves downward and the many dentitions in the aperture. Remarks. The presence of a rectangular reticulated shell sculpture and the many dentitions reaching the peristome edge where they form small denticles and extend inside the aperture, clearly position A. rhamphodontis sp. nov. within the genus Acinolaemus (Thompson and Upatham 1997). Discussion The report of these three new species from central Thailand raises some interesting questions regarding the generic boundaries and relationships between Clostophis and Acinolaemus. In the original description, Acinolaemus was characterised by an auriculate-shaped aperture, 1–3 dentitions on the parietal wall, an enlarged and conspicuous angular lamella, a well-developed sinulus (formed by angular lamella and palatal plica), and a spirally striated protoconch (Table 4; Thompson and Upatham 1997; Schileyko 1998). However, these diagnostic characters are not consistently expressed across all species assigned to the genus, casting doubt on their reliability for the diagnosis of the genus. For example, six species, i.e., A. cryptidentatus, A. dayanum, A. mueangonensis, A. ptychochilus (type species), A. rhamphodontis sp. nov., and A. corusticorus sp. nov. exhibit many dentitions that reach the peristome edge and extend deeper inside the aperture as long dentitions, a well-developed palatal tubercle, and with distinct rectangular reticulated shell sculptures (Thompson and Upatham 1997; Changlom et al. 2019; Tongkerd et al. 2024). In the present study, A. mueangonensis and A. rhamphodontis sp. nov. appeared as well-supported sister taxa, while A. sphinctinion Thompson & Upatham, 1997, A. colpodon Thompson & Upatham, 1997, A. stenopus, and A. rhamphodon possess only a few apertural 67 ZooKeys 1258: 35–71 (2025), DOI: 10.3897/zookeys.1258.162797 Piyoros Tongkerd et al.: The microsnail genera Clostophis and Acinolaemus from Thailand dentitions and a palatal wall with one or two lamellae. Our phylogeny indicates that A. rhamphodon and A. colpodon form a clade with C. rhynchotes sp. nov. and C. udayaditinus, with support from the ML analysis. Interestingly, these four species share certain morphological characters such as prominent spiral striations relative to the growth lines and few apertural dentitions, which are the distinguishing characters of the Clostophis (Páll-Gergely et al. 2020). Moreover, three other species, namely A. carcharodon, A. pyramidalis and A. rectus, possess a short to long tuba, four or five apertural dentitions, and prominent spiral striations (Vermeulen et al. 2007, 2019) and these conchological characters suggest a close relationship between Acinolaemus and Clostophis, with this latter genus being defined by a colourless shell, short to long tuba, prominent spiral striations, and typically with one or more apertural dentitions (Páll-Gergely et al. 2020). Previous DNA studies by Tongkerd et al. (2004) suggest that the shell surface sculpture provides a more informative and reliable phylogenetic signal than shell shape or dentition, and this was taken into consideration in the Gojšina et al. (2025) revision. However, due to the limited number of genera and species examined in the Hypselostomatidae, it remains premature to expand further on the relationship between Clostophis and Acinolaemus. Acknowledgements The authors are grateful to all members of the Animal Systematics Research Unit (ASRU), Chulalongkorn University, for their kind help during field trips. We also thank to E. Jeratthitikul for the map. We also thank DJ Anderson for his initial linguistic help in the original manuscript. We gratefully acknowledge the two reviewers, T. Backeljau and B. Páll-Gergely for their helpful and constructive comments, which significantly improved the manuscript. This study was conducted following the approved animal use protocol by the Animal Care and Use Committee of the Faculty of Science, Chulalongkorn University (Protocol Review Nos. 2423003 and 2523009). Additional information Conflict of interest The authors have declared that no competing interests exist. Ethical statement No ethical statement was reported. Use of AI No use of AI was reported. Funding This research is funded and supported by the NSRF via the Program Management Unit for Human Resources & Institutional Development, Research and Innovation (B42G670038) to SP; National Research Council of Thailand (NRCT-N35E660138 and NRCT-N34E670115) and Centre of Excellence on Biodiversity (MHESI) to SP and CS; grants for development of new faculty staff, Ratchadapiseksompotch Fund, Chulalongkorn University to AP; grants of Ratchadapiseksompotch Fund Chulalongkorn University, 68 ZooKeys 1258: 35–71 (2025), DOI: 10.3897/zookeys.1258.162797 Piyoros Tongkerd et al.: The microsnail genera Clostophis and Acinolaemus from Thailand and the Thailand Science Research and Innovation Fund Chulalongkorn University to PT; the Serbian Ministry of Science, Technological Development and Innovation (Contract No. 451-03-136/2025-03/ 200178) to VG. Author contributions PT, TJ, AP, SP, and CS collected and prepared specimens in the field. TJ and AP performed molecular data analysis and interpreted the analysis results. PT, AP, VG, and CS provided financial and intellectual support. PT, TJ, AP, and CS prepared specimens and wrote the manuscript. PT, SP and CS conceived, designed, supervised the study, prepared figures, and approved and edited the final manuscript. VG, SP, and CS reviewed, advised, and approved the final manuscript. Author ORCIDs Piyoros Tongkerd https://orcid.org/0000-0001-9221-9293 Teerangkul Janjai https://orcid.org/0009-0007-1524-9668 Arthit Pholyotha https://orcid.org/0000-0001-6677-1164 Vukašin Gojšina https://orcid.org/0000-0002-0413-9304 Somsak Panha https://orcid.org/0000-0002-4431-2458 Chirasak Sutcharit https://orcid.org/0000-0001-7670-9540 Data availability All of the data that support the findings of this study are available in the main text. References Benson WH (1860) On Clostophis and Rhiostoma, new Burmese genera of landshells. 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