Pincerna Preston, 1907

Pall-Gergely, Barna, Sajan, Sheikh, Tripathy, Basudev, Meng, Kaibaryer, Asami, Takahiro & Ablett, Jonathan D., 2020, Genus-level revision of the Alycaeidae (Gastropoda, Cyclophoroidea), with an annotated species catalogue, ZooKeys 981, pp. 1-220 : 1

Genus Pincerna Preston, 1907

Alycaeus (Pincerna) Preston, 1907: 206.

Alycaeus (Cycloryx) Godwin-Austen 1914: 334.

Chamalycaeus (Cycloryx) - Thiele 1929: 108; Wenz 1938: 478.

Alycaeus (Pincerna) - Thiele 1929: 108; Wenz 1938: 479; Egorov 2013: 33-34.

Chamalycaeus (Cycloryx) - Egorov 2013: 36.

Pincerna - Páll-Gergely 2017: 214.

Type species.

Pincerna liratula Preston, 1907 (Fig. 39B ) (introduced as a subgenus of Alycaeus , but apparently used on genus level), by monotypy.

Diagnosis.

Shell very small to medium sized (D: 2.5-6 mm), globose triangular, usually higher than wide; protoconch smooth, without spiral striation; R1 usually with strong, widely-spaced ribs and weak spiral striation; R2 smooth to prominently ribbed (in those cases not different from R1), very short, pyriform (in typical Cycloryx species) or somewhat longer in Pincerna -like shells; umbilicus often closed by the reflected columellar extension in many species. Operculum thin, in some species with a low circular structure, or calcareous ridges radiating outwards of the nucleus on the outer surface. Radula of a single species is known, see Table 1 (central tooth with 5 cusps, broad, central cusp pointed).

Differential diagnosis.

See Remarks.

Distribution.

The distribution of Pincerna seemingly consists of two disjunct geographic areas, namely the southeastern Himalaya to northern Vietnam and northern Laos, and Sumatra, the southern part of the Malay Peninsula, and northern Borneo (Fig. 40 ).

Remarks.

The relationship between Alycaeus , Pincerna , and Cycloryx is the most problematical point of the current revision. The genus Cycloryx Godwin-Austen, 1914 (type species: Cyclostoma constrictum Benson, 1851, OD) was erected as a subgenus of Alycaeus Gray, 1850, and was described on the basis of the ovately conoid shell shape, the regular ribbing on the upper whorls, and the extremely short, often clubbed or pear-shaped sutural tube (Godwin-Austen 1914). Godwin-Austen (1914) only included species from northeastern India and Burma (Rakhin = Arakan, and the Shan States) in this genus. However, the diagnosis of Cycloryx matches several species extralimital to the distributional range as defined by Godwin-Austen, namely those from northern Vietnam, Borneo, China’s Guizhou Province, the Malay Peninsula, and Sumatra. The most striking example is the Sumatran Pincerna yanseni and the northern Vietnamese Alycaeus costulosus , which both look so similar to Indian and Burmese species that it was challenging to find any differences between those two and the Indian and Burmese taxa. One of those "extralimital Cycloryx " is Alycaeus liratulus Preston, 1907, known from the Malay Peninsula and Sumatra, which has been placed in its own subgenus, Pincerna Preston, 1907. Originally, Pincerna was diagnosed on the basis of a "circular cup" on the outer surface of the operculum. The outer surface of operculum, however, has limited taxonomic value at the generic level in the Alycaeidae , especially since the outer rings have evolved in multiple alycaeid genera ( Dicharax , Metalycaeus , Stomacosmethis ; see this study and Páll-Gergely et al. 2017). Moreover, the most similar species to P. liratula , P. thieroti , lacks the circular cup on the outer surface of the operculum. Consequently, no important conchological characters distinguish Cycloryx and Pincerna , and thus, they have been synonymised ( Páll-Gergely 2017). See also under Genus-level diversity (p. 13).

Pincerna is globular and sparsely ribbed, Stomacosmethis is triangular and densely ribbed. Stomacosmethis balingensis is globular, but densely ribbed, forming a connection between the two genera, but the radula morphology unambiguously indicates its position within Stomacosmethis . We retain the two genera separate due to the unique radular morphology of all known Stomacosmethis species. Furthermore, there are many species with typical features of both respective genera. See also under Alycaeus (p. 17).

Figures

Figure 39. Type species of alycaeid genus-group taxa A Pincerna constricta (Benson, 1851), syntype (NHMUK 1906.4.4.41; type species of Cycloryx) B Pincerna liratula (Preston, 1907), syntype (NHMUK 1907.5.20.191; type species of Pincerna) C Stomacosmethis sarasinorum (Bollinger, 1918), lectotype (NHMB 2411. a; type species of Stomacosmethis). Photographs: Harold Taylor (A, B); Eduard Stoeckli (C).

Figure 40. Distribution of Pincerna Preston, 1907 (dark shaded area) and Stomacosmethis Bollinger, 1918 (light shaded area).