Hydrobioides physcus Annandale, 1918 comb. rev. Hydrobioides physcus Annandale, 1918: 121–122, pl. 13, figs 8, 8 a, 9, pl. 14, figs 5, 5 a (Myanmar, Shan State, Inle Lake). Hydrobioides (Paranerita) physcus – Annandale, 1920: 45. Paranerita physcus – Annandale & Rao, 1925: 115. Parabithynia physcus – Pilsbry, 1928: 108 Gabbia physcus – Subba Rao, 1989: 78, figs 127–129. Diagnosis. Shell medium, low spiral whorls, weak keel on shoulder, outer lip outward extended. Material examined. • 3 probable paratypes (preserved dry), Inle Lake, South Shan, Burma. “ Ex. India Museum ”, NHMUK.20060144; 2 specimens (preserved in ethanol), Shan State, Taunggyi District, Inn Paw Hkon, Inthein, riverbank under Inthein Bridge (Fig. 1 B. d), 20°27'35.9"N, 96°50'32.6"E, 1 Jul. 2024, Le-Jia Zhang leg., KIZ.2400061 –2400062 • 2 specimens (preserved in ethanol), Shan State, Taunggyi District, Nyaungshwe, Inle Lake, floating islands near Intha (Fig. 1 B. c), 20°36'33.4"N, 96°54'53.8"E, 3 Jul. 2024, Le-Jia Zhang leg., KIZ.2400063 –2400064. Description. Shell (Fig. 7) medium for the family (Table 1), relatively thick, solid, subglobose to globose, white or bright yellow to orange yellow, with four whorls at adulthood; teleoconch low, with a keel on shoulder, forming weak nodules on shoulder with vertical growth lines, two weak additional keels on body whorl; aperture ovate, lip thickened, always more than half of shell in height, outer lip outward extended; umbilicus covered by inner lip completely. Operculum (Figs 7 A, 4 B) calcium, ovate, thin, slightly smaller than aperture, transparent grey; exterior surface smooth, nucleus close to the central of operculum; interior surface with relatively thin outer opercular region, narrow weak concentric veins on the margin of inner opercular region, nuclear region relatively large, with irregular veins and grains. Penis (Fig. 5 B) with a white penial appendix located in the central of penis, penial appendix slightly shorter than distal part of penis. Radula (Fig. 8) taenioglossate; central tooth upper margin with one broad triangle central cusp and two to three small sharp cusps on either side, lateral margin each with two to three small sharp cusps; lateral tooth with one broad tongue-shape central cusp and two to three small sharp cusps; inner marginal tooth with 10 to 11 small cusps; outer marginal tooth with six small cusps. Remarks. This species can be easily distinguished from other bithyniid snails based on the thick subglobose to globose shell with keels and shoulder. It also can be distinguished from H. nassa based on opercular characters and less small cusps on inner and outer marginal teeth. This species is the type species and only extant species of genus Parabithynia Pilsbry, 1928, which is now endemic to Inle Lake. Several fossil species of this genus were reported from China and Russia (Popova 1981; Yu et al. 1978), but the assignment needs a revision. Our molecular study confirms that this species is the sister species of H. nassa, with a close genetic relationship (p distance of 16 S: 4.3–4.5 %). The similarity in shell (especially the extended outer lip), operculum, and male genital structure also supports that these two species should be placed in one genus. Therefore, we consider that Parabithynia is a synonym of Hydrobioides. Hydrobioides physcus comb. rev. should be re-assigned to its original genus. Habitat and distribution. Inle Lake and the big rivers connected to Inle Lake; swamps in Heho, Shan State, Myanmar.

Published as part of Zhang, Le-Jia, Shu, Shu-Sen, Song, Xin-Yuan, Naing, Nay Htet, Oo, Thaung Naing & Chen, Xiao-Yong, 2025, A revision of Bithyniidae (Mollusca, Gastropoda) from the Inle Lake Basin, Myanmar, pp. 643-660 in Zoosystematics and Evolution 101 (2) on pages 643-660, DOI: 10.3897/zse.101.143936