Reassessment and systematic position of the sinistral snails of genus Hemiplecta from Thailand (Eupulmonata: Ariophantidae), with description of two new species


Indochina land snails of the family Ariophantidae are in need of thorough systematic revision. Here we comprehensively revise the systematics of the large-shelled, sinistral (counterclockwise) coiling snails from Thailand and Peninsular Malaysia. Molecular phylogeny based on mitochondrial (coi and 16S) and nuclear (28S) gene sequences demonstrates that these sinistral snails are not members of Dyakiidae as previously thought, but instead are more closely related to the genus Hemiplecta in the family Ariophantidae. Comparative morphology also reveals similarity of reproductive organ features (globular gametolytic organ, well-developed dart apparatus, and lack of amatorial organ complex). Based on this evidence, we propose to transfer these sinistral snails to the genus Hemiplecta. Molecular phylogenetic analyses further strongly support the monophyly of this sinistral lineage with respect to other members of Hemiplecta. This monophyletic clade consists of five members including three species that were previously classified as “Dyakia”, H. lahatensis, H. retrorsa and H. salangana, and two new species described herein, H. ligorica n. sp. and H. thailandica n. sp. This study also suggests that the anatomy of the gametolytic organ in the genus Hemiplecta corresponds well with the phylogenetic relationships and appears to be a taxonomically informative character, while the penial verge has little utility for generic recognition.


Introduction
Southeast Asia has a diverse land snail fauna, and species vary greatly in size; shell diameter varies from half a millimeter (hypselostomatid; Schileyko, 1998;Páll-Gergely et al., 2015) to nearly seventy millimeters (i.e., ariophantids and dyakiids; Schileyko, 2002Schileyko, , 2003. Traditionally, land snail classification has relied mainly on shell traits, and to a lesser extent has been based on reproductive anatomy. Several recent systematic studies have revealed considerable plasticity in morphological characteristics, and substantial incongruence between the morphology-based classification and molecular phylogenetic analyses. Such findings fuel the controversy over whether standard taxonomic methods are inadequate, and whether the distinct phylogenetic lineages derived from molecular data could facilitate the task of biological classification (Seberg et al., 2003;Tautz et al., 2003;Dunn, 2003;Hebert & Gregory, 2005;Hirano et al., 2014;Köhler & Criscione, 2015;Pholyotha et al., 2020).
In the present work, we revise the systematics of large-sized helicoid species assigned to the genera Hemiplecta Albers, 1850and Koratia Godwin-Austen, 1919(family Ariophantidae Godwin-Austen, 1883, as well as Dyakia Godwin-Austen, 1891 (family Dyakiidae Gude and Woodward, 1921) from Indochina. The genus Hemiplecta is widely distributed from India to Southeast Asia. This genus is characterized by a large shell and by globular shape of the gametolytic sac (Blanford & Godwin-Austen, 1908;Schileyko, 2002Schileyko, , 2015. The genus Koratia was treated as a subgenus of Hemiplecta by Zilch (1959) and Solem (1966), but was later elevated to genus rank by Schileyko (2002Schileyko ( , 2015 based on the absence of a penial verge in the reproductive organ. The genus Dyakia, which was described to include several Greater Sunda Islands species, is characterized by both sinistral (counterclockwise) shell and the unique genitalia characters of the amatorial organ complex (Godwin-Austen, 1898;Schileyko 2003;Sutcharit et al., 2012Sutcharit et al., , 2019. Godwin-Austen (1898) was the first to revise the systematics of these snails and found that the sinistral species from the Malay Peninsula (Myanmar and Thailand) exhibit a distinct shape of the radula teeth. Laidlaw (1963) revised the taxonomy of Dyakia, but was uncertain about the Malay Peninsula species as they exhibited distinctive morphological character states (sculpturing as spiral wrinkles). Since then, the Southeast Asian helicoid snails with large and sinistral shells have consistently been attributed to the genus Dyakia s.l. (e.g., Panha, 1996;Maassen, 2001;Sutcharit et al., 2012).
However, this largely shell-based classification has never been tested in a phylogenetic context. Given that shell characteristics have repeatedly been found to evolve convergently in a wide range of land snail taxa, such a phylogenetic analysis is crucial in creating a natural classification. The recent morphological revision of so-called "Dyakia" from Peninsular Malaysia clearly revealed that this grouping was polyphyletic, with Helix retrorsa Gould, 1843 andNanina salangana Martens, 1883 found to display genitalia similar to Hemiplecta (Sutcharit et al., 2012). This contradiction indicates that most of the shell characteristics vary considerably within the higher taxonomic categories and thus are unreliable for classification. In addition, a large proportion of Indochinese land snail taxa are still known exclusively from their shells. The high similarity of shell morphology does not always indicate a common ancestry, but rather it may frequently reflect convergent evolution related to adaptations to varying environmental conditions (Vermeij, 1993;Hirano et al., 2015;Köhler & Criscione, 2015), predation (Schilthuizen et al., 2006) or competition (Oheimb et al., 2018). Rather than shell features, the characters of the genital anatomy have been shown to be particularly informative for the recognition of taxa in many cases of the stylommatophoran groups, such as in the genera Macrochlamys Gray, 1847, Sarika Godwin-Austen, 1907and Traphospira Blanford, 1905(Pholyotha et al., 2018, 2020. Convergent shell features have also been found in many cases among the Australian camaenids (Köhler, 2010(Köhler, , 2011a(Köhler, , b, c, 2012Köhler & Shea, 2012).
Land snail surveys from southern Thailand and Peninsular Malaysia have yielded a number of the sinistrail snails that were preliminarily classified as the genus "Dyakia" based on their shell coiling. However, this classification has remained provisional. Better sampling and more data are needed to determine whether these sinistral species are members of Hemiplecta or Dyakia. Therefore, the primary objective of this study is to test the correspondence between traditional morphology by means of comparative study of key morphological characters (i.e., shell and genital anatomy) and the monophyletic groups based on molecular phylogeny of these sinistral snails. The proposed taxonomic rearrangement of Hemiplecta and Dyakia and descriptions of two new species are described herein, along with re-descriptions of the other three nominal species.

Sampling and morphological examination
Snails were sampled throughout eastern to southern Thailand and Peninsular Malaysia ( fig. 1). The details of each locality are listed in table 1. Animal use protocol in this study was approved by the Chulalongkorn University Animal Care and Use Committee (cu-acuc) under the approval number 1723018. Living snails were euthanized by a two-step method following avma Guidelines for the Euthanasia of Animals (AVMA, 2013). Foot tissues were cut and stored in 95% ethanol before use for dna extraction. Other remaining parts were preserved in 70% (v/v) ethanol for anatomical purposes. The genital organs of 3-5 specimens of each species were examined. Radulae were extracted, and examined under Scanning Electron Microscope (jeol, JSM-5410 LV). The radula shape and teeth formula were analyzed.
figure 1 Distribution of the sinistral Hemiplecta species examined in this study.

Molecular studies
Up to three specimens from each population and each recognizable morph were selected for dna sequencing, with priority given to dissected individuals. Genomic dna was extracted from the foot or mantle tissue using a NucleoSpin Tissue kit (macherey-nagel), following the manufacturer's protocol. The mitochondrial cytochrome c oxidase subunit I gene (COI) was amplified using the universal primers LCO1490 -G G T C A AC A A AT C ATA A AG ATAT T G G and HCO2198 -TAAACTTCAGGGTGAC-CAAAAAATCA (Folmer et al., 1994), while the 16S ribosomal rna gene (16S) was amplified by primers 16SsinF -GCCGCAGTACCTTGACTGTGCT or 16Sar -CGCCTGTTTATCAAAAACAT with 16sbr -CCGGTCTGAACTCAGATCACGT (Kessing et al., 1989), and the nuclear 28S ribosomal rna gene (28S) was amplified by primers 28SF4 -AGTACCGTGAGGGAAAGTTG and 28SR5 -ACGGGACGGGCCGGTGGTGC (Morgan et al., 2002). The pcr amplification was conducted in a final volume of 20 µl containing 1 µl of each primer (10 mM), 2 µl of the extracted genomic dna (25 ng), 7 µl of distilled water, and 10 µl of EmeraldAmp pcr Master Mix (TAKARA BIO INC.). Thermal cycling was performed at 94°C for 3 min, followed by 35 cycles of 94°C for 30 s, 51-60°C (depending on samples and gene) for 60 s, extension at 72°C for 1 min, and then the final extension at 72°C for 5 min. All pcr products were purified by peg precipitation and then commercially sequenced by an automated sequencer (abi prism 3730XL) with the same primers in both directions. Newly obtained nucleotide sequences were deposited in the GenBank database (see table 1 for accession numbers).

Phylogenetic analysis
Sequences were edited and aligned using ClustalW, implemented in MEGA7 (Kumar et al., 2016). The final concatenated alignment used in phylogenetic tree construction contained 1576 bp: 655 bp of COI, 369 bp of 16S, and 552 bp of 28S. These sequences came from 43 specimens representing seven genera of Dyakiidae and four recognized genera of Ariophantidae occurring in Thailand, along with specimens of the non-limacoid taxa Camaenidae and Helicidae as outgroups. The details of taxon sampling used in phylogenetic analysis are shown in table 1. Most of the sequences in this study were newly obtained from 42 specimens, but some sequences were retrieved from previous studies (i.e., Sutcharit et al., 2019 andPholyotha et al., 2020).
The best-fit model of nucleotide substitution and the best partitioning scheme were identified by PartitionFinder2 v.2.3.4 (Lanfear et al., 2016), by using a heuristic search algorithm under the Akaike Information Criterion (aic). The partitioned concatenated dataset was then used to infer the phylogenetic relationship by using Bayesian inference (bi) and maximum likelihood (ml) analyses, implemented in cipres Science Gateway (Miller et al., 2010). The ml analysis was performed using RAxML v.8.2.10 (Stamatakis, 2014). The GTRGAMMA was set as the model for all gene partitions. One thousand ml bootstrap replicates were performed to assess topology support. The bi trees were estimated by running a 10 million generation Metropolis-coupled Markov chain Monte Carlo (mc-mcmc) as implemented in MrBayes 3.2.6 (Ronquist et al., 2012). The best-fit models for each partition followed the PartitionFinder2 results. Each mcmc consisted of two runs with four chains, one chain of which was heated. A data systematics of land snails genus hemiplecta | 10.1163/18759866-bja10016 partition was applied that allowed parameters to be estimated separately for each partition. Sampling rate of the trees was once every 1000 generations. Stationarity was considered to have been reached when the average standard deviation of split frequencies shown in MrBayes was less than 0.01 and the log likelihood of sampled trees reached a stationary distribution. The first 25% of obtained trees were discarded as burn-in. The remaining trees were used to estimate the consensus tree topology, bipartition posterior probability (bpp) and branch lengths. Nodes with bipartition posterior probabilities (bpp) greater than 0.95 of bi analysis supported and/or bootstrap support values larger than 70 of ml analysis were considered as supported nodes (Huelsenbeck & Hillis, 1993;Larget & Simon, 1999). Genetic divergence based on COI sequences among pairwise OTUs was also calculated using uncorrected p-distances as implemented in MEGA7 (Kumar et al., 2016).

Results
Molecular phylogenetic trees were reconstructed based on the concatenated mitochondrial COI and 16S genes, and as well as nuclear 28S sequences obtained from 43 individuals. This concatenated dataset was divided into five partitions, consisting of partitions for 16S and 28S genes, and for each of three codon positions of the COI gene. The best-fit models for each partition were GTR+G for the first codon position of COI and 16S, GTR+I+G for the second codon position of COI and 28S, and GTR+I for the third codon position of COI genes. Both maximum likelihood (ml) and Bayesian inference (bi) analyses produced trees with identical topologies, and with all major clades relatively well supported. Therefore, only the tree topology from bi is presented in fig. 2. Overall, our phylogeny is largely congruent with the current generic and familial classification with the notable exception that some sinistral species currently treated as members of the genus Dyakia (Dyakiidae) cluster closely together with species of the genus Hemiplecta (Ariophantidae). However, these two families are distinctly recognized by the comparative morphology of shells and genitalia.
All nominal genera that were placed in the Dyakiidae based on their morphology are recovered as one major clade in our tree. However, Dyakia, as presently delineated, is non-monophyletic. While D. janus (Beck, 1837) is nested with the dyakiid clade, five other species assigned to this genus are placed among the Ariophantidae, and more specifically, within the genus Hemiplecta. The relationships among other dyakiid genera, mostly represented by the type species, are well supported in both bi and ml analyses. Three sinistral genera, Dyakia, Rhinocochlis Thiele, 1931 and Bertia Ancey, 1887 are not recovered as monophyletic. All three species of the Phuphania Tumpeesuwan et al., 2007 form the sister group to Quantula striata (Gray, 1834). The clade containing the ariophantid genera, Cryptozona Mörch, 1872, Macrochlamys and Sarika forms a sister group to Dyakiidae, with strong support from both ml and bi analyses.
The other major clade consists of the remaining ariophantids, including the genera Hemiplecta and Koratia, and the five sinistral snails that were provisionally classified as "Dyakia" species from southern Thailand and Peninsular Malaysia. Three of these have previously been identified as nominal species, "Dyakia" retrorsa, "Dyakia" salangana, and "Dyakia" lahatensis (Morgan, 1885a), whereas the other two represent putative newly discovered species. The phylogenetic tree also reveals that all these five sinistral species form a monophyletic clade and that 10.1163/18759866-bja10016 | sutcharit et al. this clade is sister to the other members of the genus Hemiplecta and Koratia. Within this clade, "Dyakia" retrorsa is most closely related to "Dyakia" sp. 1 from Nakhon Si Thammarat, Thailand. "Dyakia" salangana is sister to "Dyakia" lahatensis, while "Dyakia" sp. 2 from eastern Thailand is the sister lineage of the above four species.
The genus Koratia (type species: Helix distincta Pfeiffer, 1850) does not form a monoplyletic clade based on the present analysis. "Koratia" distincta and "Koratia" pluto (Pfeiffer,  Within the genus Hemiplecta, the average genetic distance was estimated to be 8.14% (table 2). The minimum divergence is 2.92% between the pair of H. cymatium (Pfeiffer, 1856) and H. humphreysiana, while the maximum divergence is 9.75% between "Dyakia" salangana and H. humphreysiana. Among the five sinistral "Dyakia" snails, the average genetic distance ranges from 5.83% to 7.46%. The intraspecific genetic distances for these species are relatively low, except in "Dyakia" salangana, where the structured subclades show a considerable divergence of 3.03% between Thai and Malaysian populations.
All systematic descriptions and related remarks are presented as Appendix.

Evolution of the sinistral clade within the dextral-dominated genus
Current systematic classification of the Dyakiidae and Ariophantidae is still heavily influenced by traditional shell-based characters, and is sometimes incongruent with molecular phylogeny (Hyman et al., 2007(Hyman et al., , 2017Köhler et al., 2020). Diagnostic characters of reproductive organs in Dyakiidae are the presence of amatorial organ complex (amatorial organ gland and duct) and the connection of the gametolytic organ to the amatorial organ (Laidlaw, 1931;Hausdorf, 1995;Sutcharit et al., 2012). However, unique characters of the Ariophantidae are still equivocal. Most of the known taxa are described based solely on their shells, and without genitalia characters. Moreover, a conclusive phylogeny is still far from complete. Nevertheless, the currently accepted defining character states of the Ariophantidae include the presence of shell lobes, caudal foss, and well-developed caudal horn, either absence or presence of amatorial organ (but if present, without accessory organs), and a gametolytic organ that is attached to the vagina (Hausdorf, 1998;Schileyko, 2002;Hyman & Ponder, 2010). The present phylogeny reveals that five sinistral species are clustered with the dextrally-coiled genus Hemiplecta (Ariophantidae), although sinistrality has long been considered as a diagnostic character of Dyakia s.s. (Godwin-Austen, 1891;Laidlaw, 1931;Schileyko, 2003). This phylogenetic grouping is supported by the anatomy of reproductive organs. The genitalia of sinistral clade resembled those of Hemiplecta instead of the Dyakia. Only the reversed genital opening on the body side was detected between opposite coiling snails (Sutcharit et al., 2012;this study). Normally, the reproductive anatomy in stylommatophorans is highly asymmetric, and the sinistral individuals have identical genital structure as a mirror-image to other dextral individuals of the same taxa (Gittenberger, 1988).
From our multi-locus phylogeny, sinistrality thus is not a trait unique to Dyakia but has convergently evolved in Hemiplecta as well. Sinistrality has sporadically evolved and has occurred independently among different stylommatophoran clades (Asami et al., 1998;Sutcharit et al., 2007;Hoso et al., 2010;Jirapatrasilp et al., 2020). In our results, the sinistral shell coiling has become fixed within this Hemiplecta clade, similar to the Japanese endemic genus Euhadra Pilsbry, 1890, of which four out of about 20 species are sinistral and all are derived from a single sinistral ancestor (Ueshima and Asami, 2003;Davison et al., 2005). In other cases, dextral and sinistral individuals co-occur in the same population (chirally dimorphic). For example, in the Southeast Asian endemic tree snail genus Amphidromus Albers, 1850, which comprises two subgenera. One subgenus, Amphidromus, includes chirally dimorphic species, while another subgenus, Syndromus Pilsbry, 1900, includes only sinistral species. Phylogenetic analysis has shown that sinistrality has persisted as the ancestral state of the sinistral Syndromus clade (Sutcharit et al., 2007), which is similar to the case of the sinistral Hemiplecta clade in this study.
It is interesting to consider how sinistrality has evolved independently within the genus Hemiplecta, where dextrality is the dominant or more common morphology. One of the likely explanations is predation pressure. A comparative survey of the diversity of the sinistral snails as potential prey (i.e,. those with lower-spired shells) showed that the sinistral taxa occur more commonly within the distribution range of a snail-eating snake (genus Pareas) (Hoso et al., 2010). In feeding experiments, snail-eating snakes tended to prey on dextral over sinistral individuals. This survival advantage of sinistral individuals is more likely to enhance the chance of population fixation by complete premating isolation (Hoso et al., 2007(Hoso et al., , 2010Danaisawadi et al., 2016). Another predation experiment revealed that snail-eating snakes avoided approaching or striking at a sinistral snail ("Dyakia" salangana), suggesting it was more difficult and costlier to handle than dextral snails (Danaisawadi et al., 2016). The adult Hemiplecta spp. have much greater shell size than the predation threshold size; however, these large snails can be small enough to be preyed upon when they are young. The factor of snake predation may counterbalance the disadvantage in copulation, and if the sinistral phenotype exceeds 50% under selective advantage, this phenotype would become fixed for the population (Gittenberger, 1988;Johnson et al., 1990;Asami et al., 1998;Ueshima and Asami, 2003;Davison et al., 2005;Gittenberger et al., 2012). Moreover, many studies have shown that interchiral mating (copulation between individuals with opposite coiling direction) is almost impossible in low-spired snails (shell width greater than shell height) that generally perform simultaneous reciprocal copulation. Thus, reverse coiling populations would become isolated rapidly and give rise to a single-origin sinistral clade through a complete premating isolation mechanism (Batenburg & Gittenberger, 1996;Gittenberger, 1988;Asami et al., 1998;Reise et al., 2002;Gittenberger et al., 2012).

Taxonomic implications in Hemiplecta
The genus Hemiplecta currently comprises at least twenty nominal species, with most known by shell characteristics only (Schileyko, 2002). The anatomy of the type species, H. humphreysiana, was described by Collinge (1902) and Blanford & Godwin-Austen (1908), and that of H. cymatium and H. distincta was described by Stoliczka (1873), Godwin-Austen (1900, 1919, and Schileyko (2002). The conic to depressed-conic shell, globular gametolytic sac with undifferentiated gametolytic duct, very short flagellum, and short to long epiphallic caecum have been recognized as characters unique to the genus (Schileyko, 2002(Schileyko, , 2015. The function of the epiphallus and epiphallic flagellum is thought to produce the spermatophore, the tail-pipe forming in the flagellum, and the capsule in the epiphallus. The epiphallic caecum is believed to function in turning the spermatophore so that the tail-pipe is moved into the penis first (Gόmez, 2001).
The nominal genus Koratia was previously treated as a distinct genus based on the absence of a penial verge inside the penis (table 3; Godwin-Austen, 1919; Schileyko, 2002). However, our phylogenetic analysis indicates that three nominal species  (Davison et al., 2009). The deep divergence detected here is similar in order of magnitude to that in other land snails reported elsewhere; for example, about 6-12 % among Australian camaenid species (Criscione et al., 2012). Furthermore, the molecular phylogenetic results also confirm the validity of H. funerea as a genetically distinct species and undoubtedly support the specific validity of H. pluto. Traditionally, these two species have been recognized as a subspecific entity or a synonym, respectively, of H. distincta (Fischer & Dautzenberg, 1904;Schileyko, 2011). Only the recent treatment by Inkhavilay et al. (2019) has raised them to a specific level, but without clear supporting evidence.
Currently, the sinistral clade comprises five nominal species, four of which were anatomically examined, with the exception of H. lahatensis. This species possesses a sinistral shell with a wrinkled shell surface, and its distribution covers the northern part of Peninsular Malaysia; it likely belongs to the Hemiplecta.
Further examination of newly-collected adult specimens are necessary to confirm this species' generic position and morphological relationship with other sinistral-shelled taxa.
In terms of distribution, four of the species range from the southern Tenasserim Range through the Malay Peninsula. The exception is H. thailandica n. sp., which is restricted to Indochina (Eastern Thailand) and probably in Cambodia ; this could be attributed to allopatric speciation. This close relationship between the Malay Peninsula and Indochina populations has been similarly reported in various land snail groups and also in other animals, including butterflies and centipedes (Muadsub & Pinkaew, 2014;Siriwut et al., 2015). These repeated disjunct distribution patterns are indicative of a cause that is linked to the geographical history of Sundaland. The fluctuation of sea level in the South China Sea contributed to the exposure of rainforest connection between Indochina and the Malay Peninsula (Voris, 2000;Woodruff, 2010;Lukoschek et al., 2011).
The most consistent and reliable character for species delimitation in every species of land snail is the internal anatomy of the penis, along with other genital characters relating to the penis and epiphallus, whereas the gametolytic organ was informative at the genus level (Hyman et al., 2017;Pholyotha et al., 2020;Siriboon et al., 2020). For example, reproductive organs are highly consistent in the dark shell and yellow shell forms (C. Sutcharit, unpublished data) of H. funerea, (i.e., coiled epiphallic caecum and penial verge absent; table 3). This supports the hypothesis that reproductive anatomy is an important factor in species recognition during copulation. In contrast, when delineating species by their shell morphology, the shape and coiling are of little value. The shell banding patterns are readily recognizable in some living species, e.g., H. ligorica n. sp., by its small and banded shell. However, this feature may be more difficult to distinguish in bleached, dead shells. Another pair of species that co-occur, H. pluto and H. distincta, can reliably be distinguished in life by their red or brownish body color, respectively (see Inkhavilay et al., 2019: fig. 56c, d), although the colors are faded in alcohol-preserved specimens.

Conclusion
Based on phylogenetic results and morphological information in the present study, we have transferred the sinistral species previously and incorrectly assigned to Dyakia (family Dyakiidae) to the genus Hemiplecta (family Ariophantidae). Two previously unrecognised species are described based on consistent differentiation in molecular, shell, and genitalia characters. Our data also provide further support for three other sinistral Hemiplecta species, which are also re-described herein. The results also suggest the synonymization of the formerly recognized as (sub)genus Koratia under the Hemiplecta. Finally, the results indicate the evolutionary instability of shell traits that are often used for land snail classification, while genital characters are highlighted as reliable taxonomic markers for delimiting species and at least some higher taxa. The integration of multiple independent characters including molecular evidence is crucial for delimiting higher systematic levels.
(NHM, London), R. Janssen, K.-O. Nagel and S. Hof (SMF, Frankfurt), T. von Rintelen and C. Zorn (ZMB, Berlin) for allowing the authors to examine the material housed in the type collections, the type material database, and photographs. This project was mainly funded through grants received from the Plant Genetic Conservation Project under the Royal Initiative of Her Royal Highness Princess Maha Chakri Sirindhorn, Chulalongkorn University, the trf Strategic Basic Research DBG 6080011 (2017-2019) and the Thailand Research Fund (TRF-DPG6280001). We also wish to thank the anonymous reviewers for critically reviewing the manuscript.
Remarks. This species was originally described from southeastern Myanmar, with large shell and strong peripheral keel. The specimens from southwestern peninsular Thailand agree well with the type specimens of this species. In Thailand, this species is known mainly from the western region along the Tenasserim Range in Kanchanaburi, Prachuap Khirikhan, Chumphorn, Ranong, Phang Nga and Suratthani Provinces. This species can be distinguished from H. salangana by its angular last whorl with strong keel on periphery, and without a spiral band. In contrast, H. salangana tends to have a rounded last whorl, and usually has a narrow brownish spiral band on periphery.  : Laidlaw, 1931: 191. Laidlaw, 1933: 226. Benthem Jutting, 1960: 17, 20. Berry, 1963: 14, pl. 9, fig. 61. Laidlaw, 1963: 145, 146. Chang, 1996 fig. 1. Hemiplecta salangana var. martensi Collinge, 1903: 209.  Genitalia. The external genital organs were described in Sutcharit et al. (2012: fig. 2a, b). Internal wall of penis exhibits closely packed papillae distally that abruptly cease proximally, close to atrium; penial verge absent.
Internal sculpture of vagina with thin and smooth longitudinal sculpture; internal surface of amatorial organ smooth.
Vagina long, about same length as penis, cylindrical and with a similar diameter as the penis. Internal wall of vagina corrugated, forming a thin and smooth longitudinal pilasters. Dart apparatus a very long and enlarged muscular cylinder; dart papilla short, conic with smooth surface. Gametolytic organ large, of globular shape. Free oviduct long and surrounded by thickened blackish (pinkish in fresh specimens) tissue around the middle.
Remarks. This new species is mainly distributed in the eastern part of Thailand in Chanthaburi and Trat Provinces, and probably in the Cardamom Mountains of Cambodia . Hemiplecta thailandica n. sp. can be distinguished from H. retrorsa and H. salangana in having a relatively larger shell, more elevated spire, more depressed suture, a well-rounded last whorl, with dark brown spiral band on periphery, which is only slightly descending near aperture.
Diagnosis. Small, sinistral helicoild shell with whitish color, rounded last whorl with dark brown to brownish spiral band on periphery and upper shell surface. Penial sculpture with small papillae arranged over about half of penis length.
Description Shell ( fig. 4D, E). Shell small (width up to 32.0 mm, height up to 20.0 mm), depressed conic, thickened and sinistral. Whorls 5-6, increasing regularly, little convex, with very wide and depressed suture. Spire convex; apex acute; embryonic shell smooth; following whorls with thin growth lines and spirally undulated sculpture. Periostracum thin and transparent. Shell color whitish to pale brown, usually with wide blackish to dark brown peripheral band just below periphery, and narrow brown spiral band on upper part of last whorl. Last whorl usually well-rounded to little angular. Aperture little descending, ovate-lunate; lip simple to thickened in adult specimens. Columella slightly dilated; parietal callus thin and translucent. Umbilicus narrowly open and deep.
Genitalia ( fig. 5E, F). Atrium very short. Penis a short, slender and cylindrical tube. Epiphallus long, about three times the penis length. Epiphallic caecum short, straight; penial retractor muscle thin and attached to the tip. Flagellum very short. Vas deferens a narrow tube connecting the free oviduct and epiphallic-flagellum junction. Internal wall of penis exhibits closely packed papillae over about half of penis length that abruptly cease close to atrium; penial verge absent.
Vagina long, about two times the length of penis, cylindrical and its diameter similar to that of penis. Internally vagina possesses few corrugated pilasters, which are thin and smooth longitudinal pilasters. Dart apparatus a long muscular cylinder; dart papilla long, conic, with a smooth surface. Gametolytic organ large, of globular shape. Free oviduct long and surrounded by thickened blackish tissue (pinkish in fresh specimens) at distal end.
Remarks. Hemiplecta ligorica n. sp. is known from several localities in limestones outcrops in Suratthani and Nakhon Sri Thammarat Provinces. They tend to be distributed along the Khao Luang range in moist to dry evergreen forest. This new species differs from all other sinistral species in having a relatively smaller, pale brownish to whitish shell with two dark brown spiral bands: one on the periphery and another below the periphery.