Acinolaemus dayanum (Stoliczka, 1871) Tongkerd & Lwin & Páll-Gergely & Chanabun & Pholyotha & Prasankok & Seesamut & Siriwut & Srisonchai & Sutcharit & Panha, 2024

Tongkerd, Piyoros, Lwin, Ngwe, Pall-Gergely, Barna, Chanabun, Ratmanee, Pholyotha, Arthit, Prasankok, Pongpun, Seesamut, Teerapong, Siriwut, Warut, Srisonchai, Ruttapon, Sutcharit, Chirasak & Panha, Somsak, 2024, Contributions of a small collection of terrestrial microsnails (Pupilloidea, Hypselostomatidae) from Myanmar with description of three new species, ZooKeys 1195, pp. 157-197 : 157

Acinolaemus dayanum (Stoliczka, 1871) comb. nov.

Figs 2A , 13A

Hypselostoma dayanum Stoliczka, 1871: 172, 173, pl. 7, fig. 2. Type locality: Damotha, prope Moulmein [Kayon Hill, Dhammasa Village, Mawlamyine District, Mon State]. Hanley and Theobald 1876: 59, pl. 147, fig. 10. Pfeiffer 1876: 488. Pfeiffer 1880: 344.

Boysidia (?) Boysidia dayana . Pilsbry 1917: 205, 206, pl. 34, figs 5, 6.

Hypselostoma Pupa (Hypselostoma) dayana. Nevill 1878: 193.

Hypselostoma Pupa (Hypselostoma) dayanum. Gude 1914: 300, 301.

Material examined.

Dhammasa Cave (8 m), Mawlamyine centre NEE ca 26 km, Mon State, Myanmar (16.506715°N, 97.810763°E), leg. Otani, J.U., Okubo, K. and Hunyadi, A. 11 October 2018: HA Collection (11 shells; Figs 2A , 13A ) .

Description.

Shell turban shaped, low spire, ~ 3-4 whorls and whitish in colour. Shell height 1.2-1.3 mm and shell width 1.3-1.4 mm. Apex blunt; protoconch with fine spiral striae. Teleoconch sculptured with regularly spaced radial ribs, paralleled with strongly wrinkled and malleated pits; suture well impressed and deep. Last whorl shouldered and flattened below periphery. Peristome thickened and weakly expanded; constriction and cervical crest very weak; lip whitish. Aperture subquadrate with eight dentitions marking peristome. Parietal lamella large and long, deeper inside aperture with tall ridge; infraparietal lamella small with long and low rise. Angular lamella smaller and weaker than parietal lamella, long, low rise with slope somewhat bent. Palatal tubercle strongly developed with triangular shape. Upper-, inter-, and lower- palatal plicae small, connecting peristome, and continuous with thin and straight ridges deeper inside aperture. Basal plica small, tubercle-like. Columellar lamella strong, continuing deep inside aperture with undulated ridge. Umbilicus widely perforate, ~ 1/3 of shell width, rounded and deep.

Distribution.

Currently, this species is known only from the type locality, a limestone outcrop in Mon State, Myanmar.

Differential diagnosis.

Acinolaemus dayanum differs from all known Bensonella species in Myanmar and Thailand by its unique, turban shaped shell and teleoconch surface sculptured with malleated wrinkles and pits. In contrast, the four species from Myanmar and three species from Thailand, namely B. tamphathai (Panha & Burch, 2000), B. nabhitabhatai (Panha & Burch, 2002) and B. pangmapaensis (Panha & Burch, 2002) possess a conical shell and a teleoconch surface generally with conspicuous irregular growth lines and fine spiral striae ( Panha and Burch 2000, 2002b).

Compared to the Acinolaemus species from Thailand, A. dayanum differs by having pitted sculpture on the shell surface, weaker spiral and radial striae, a larger parietal than angular lamella, and a shouldered last whorl. Acinolaemus ptychochilus Thompson & Upatham, 1997 differs from A. dayanum by its rounded last whorl, strong tubercles on the peristomal lip, and the presence of supra- and subcolumellar lamellae. Acinolaemus colpodon Thompson & Upatham, 1997 also has a rounded last whorl, possesses a hooked columellar lamella, and lacks an interpalatal plica, a basal plica and an infraparietal lamella. In addition, A. sphinctinion Thompson & Upatham, 1997 has a shouldered last whorl with blunt periphery, a short, weakly ascending tuba, and lacks the angular lamella, the parietal fold, the basal plica and the interpalatal plica ( Thompson and Upatham 1997; Panha and Burch 2005).

Additionally, A. dayanum has a similar shell form and sculpture to the two Acinolaemus species recently described from northern Thailand. It differs from A. cryptidentatus Changlom et al., 2019, which has 4-5 whorls, strong spiral striae, and discontinuous infraparietal and angular lamellae. It is also distinct from A. mueangonensis Changlom et al., 2019 [corrected original spelling], which has 4-5 whorls, brownish shell colour, and a more prominent subcolumellar than columellar lamella.

Remarks.

The holotype (single shell mentioned in the original description) could not be located in the NHM collection, but we were able to examine recently collected topotypic shells. Originally, this species was classified in Hypselostoma and later reclassified into the genus Boysidia Ancey, 1881 (i.e., Stoliczka 1871; Pilsbry 1917). Based on the small size, the whitish shell and the strong teleoconch sculpture, we transfer H. dayanum into the genus Acinolaemus . In contrast, the genus Bensonella is characterised by a brownish shell with nearly smooth or finely, spirally striated shell ( Páll-Gergely and White 2023), whereas Hypselostoma (at least the type species and the morphologically similar and geographically close species; see Gojšina et al. 2022 and Preece et al. 2022) and Boysidia are much larger, and have brownish, finely sculptured shells. Furthermore, Hypselostoma is defined on the basis of a detached body whorl and a concrescent angular and parietal lamellae, characters which do not appear in A. dayanum .

Figures

Figure 2. A Acinolaemus dayanum, topotype specimen HA Collection from Mawlamyine Township, Mon State B Acinolaemus cryptidentatus, specimen CUMZ 14413 from Taunggyi District, Shan State. The white arrow indicates constriction, the yellow arrow indicates a cervical crest. Abbreviations: a: angular lamella, b: basal plica, c: columellar lamella, ip: infraparietal lamella, itpl: interpalatal plica, lpl: lower palatal plica, p: parietal lamella, pt: palatal tubercle, upl: upper palatal plica.

Figure 13. Synoptic view of representatives of hypselostomatid snails recorded from Myanmar A Acinolaemus dayanum, specimen HA Collection B Acinolaemus cryptidentatus, specimen CUMZ 14413 C Anauchen eotvosi, holotype NHMUK 1903.7.1.1227.1 (after Pall-Gergely 2023 a: fig. 1 c) D Angustopila occidentalis, holotype HNHM 103483 (after Pall-Gergely et al. 2023: fig. 70 a) E Angustopila elevata, specimen HNHM 103484 (after Pall-Gergely et al. 2023: fig. 13 a) F Bensonella salwiniana, specimen CUMZ 14393 G Bensonella anguloobtusa, specimen CUMZ 14400 H Bensonella taiyaiorum sp. nov., paratype CUMZ 14403 I Bensonella lophiodera sp. nov., paratype CUMZ 14405 J Clostophis sankeyi, holotype UMZC I. 103320 (after Preece et al. 2022: fig. 70 e) K Clostophis thinbowguensis, holotype MNHN-IM- 2000 - 38057 (after Pall-Gergely et al. 2022: fig. 9 c) L Gyliotrachela bensonianum, syntype NHMUK 20191141 (after Gojsina et al. 2022: fig. 4 a) M Gyliotrachela hungerfordiana, specimen NHMUK collection from Perak, Malaysia N Gyliotrachela tianxingqiaoensis, holotype IZCAS TM 025075 (after Gojsina et al. 2022: fig. 3 a) O Gyliotrachela muangon, specimen CUMZ 14410 P Gyliotrachela aunglini sp. nov., paratype CUMZ 14412 Q Hypselostoma tubiferum, specimen NHMUK 1888.12.4.17 - 22 ex. Theobald collection from Tonduong, Burma.