Discradisca semicornea

Abstract

Discradisca semicornea (Preston, 1908) (Fig. 5) Acmaea semicornea Preston, 1908: 203, fig. 46, pl. 16. Discinisca indica Dall, 1920: 279. — Cooper 1973: 5, figs 18-25, pl. 1. — Bitner et al. 2008: 281-282, figs 2B-H, n. syn. Sutura fluviatilis – Grant 1937: 322. Trimusculus semicornea – Lindberg 1990: 311 (erroneous identification). Discradisca semicornea – Raven 2021: 11-13, fig. 1, pl. 1. Pilosabia trigona – Ravinesh et al. 2021: fig. 3t (erroneous identification). non Trimusculus semicornea – Van Gemert 2010: 45-46, fig. 1 (= Trimusculus mauritianus (E. von Martens, 1880); see Van Gemert 2024). TYPE MATERIAL ILLUSTRATION. — 2021 online photographs, syntypes National Zoological Collection (Zoological Survey of India); 2021 online photographs, syntypes RBINS Virtual Collections, Brussels. ORIGINAL DESCRIPTION (Preston 1908). — “Shell semicorneous, depressedly conical, irregularly subquadrate, pale yellowish brown, sculptured with fine radiating transverse ribs crossed by concentric striae, presenting a very finely cancellate appearance; edge of peristome finely crenulate; apex simple, placed slightly to one side: anterior end of shell shortened; posterior end broadly produced. Altitude 2 mm. Diam, major 9, minor 7.75.” EMENDED DESCRIPTION. — Outline of subcircular dorsal valves (8-9 mm in diameter) more irregular than in D. borneensis n. sp. Asymmetrically low-conical in profile (3 mm high) (Figs 5D, G), posterior slope straight, anterior slope slightly convex (Fig. 5G). Smooth apex in subcentral position (Figs 5B, E), shifted posteriorly. Apex area eroded on both specimens (details of larval shell and brephic shell cannot be observed well); larval shell (protegulum) apparently smooth. External surface of dorsal valve with numerous (140-145) fine, well-pronounced but slightly irregular, tuberculate radial costellae and relatively strong, concentric growth lines. Costellae continuous and densely spaced, interspaces wider than ridges. Small irregular granules on radial costellae, more pronounced on RBINS specimen (Fig. 5E). Number of costellae increasing by intercalation towards the edges (Figs 5B, E). RBINS specimen bearing traces of survived physical damage or predation, where the growth lines blurred and radial costellae broken (Figs 5E, G). Shell edge finely crenulate (Fig. 5B, E). Internal side smooth, but traces of external costellae on the internal surface, especially along the edge of valves (Figs 5C, F). On lectotype muscle scars covered by thin white chalky deposit; on RBINS specimen lighter colour of ring-shaped muscle scars faintly visible. Shell of variable, generally pale straw colour. Lectotype lighter in colour around the apex, with alternating concentric bands of lighter and darker colours towards the edge (Fig. 5B). DISTRIBUTION. — India, Sri Lanka, Philippines, Persian Gulf, and Indonesia. REMARKS The species has long been considered a mollusc after the original description by Preston (1908), but subsequent authors placed it in various genera belonging to widely differing families of gastropods: Acmaea Eschscholz, 1833 (Acmaeidae Forbes, 1850, Patellogastropoda Lindberg, 1986) (Preston 1908); Potomacmaea Peile, 1922 (Lottiidae J. E. Gray, 1840, Patellogastropoda Lindberg, 1986) (Grant 1937); Gadiniidae J. E. Gray, 1840 (now considered a junior synonym of Trimusculidae J. Q. Burch, 1945, Heterobranchia Burmeister, 1837) (Lindberg 1990); Pilosabia Iredale, 1929 (Hipponicidae Troschel, 1861, Caenogastropoda L. R. Cox, 1960) (Ravinesh et al. 2021). Syntypes of Acmaea semicornea were deposited in the Indian Museum in Kolkata (India) and subsequently moved to the National Zoological Collection (Zoological Survey of India). The specimen illustrated online appears to be the same as the specimen figured by Preston and is here selected as lectotype. Three juveniles, also from the Andaman Islands and seen by Preston are in the Natural History Museum (London, UK; NHM 1888.12.3.1-3), considered paratypes by Lindberg (1990: 314) but not assigned such status in the original paper (Preston 1908). Two additional syntypes were apparently exchanged with Dautzenberg and are kept in RBINS (Brussels, Belgium, RBINS/Dautz-Types/525910). All these syntypes are here assigned as paralectotypes. The semicornea specimens published as gastropods are all dorsal valves; there is no information about the ventral valve in these materials. The dorsal valve characteristics of semicornea fit into the variation range of Discradisca indica Dall, 1920 (compare Dall 1920; Cooper 1973; Bitner et al. 2008).

Authors 2

1. Alfréd Dulai

   Hungarian Natural History Museum · Hungarian National Museum

   Affiliation as printed

   Department of Palaeontology and Geology, Hungarian National Museum Public Collection Centre, Hungarian Natural History Museum, H- 1083, Budapest, Ludovika Square 2 (Hungary)

2. J. G. M. Raven Aachen Research associate Naturalis Biodiversity Center

   Naturalis Biodiversity Center

   Affiliation as printed

   Research Associate, Naturalis Biodiversity Center, P. O. Box 9517, NL- 2300 RA Leiden (The Netherlands)