Nanogyra Beurlen, 1958

Koppka, Jens, 2015, Revision of the Bivalvia from the Upper Jurassic Reuchenette Formation, Northwest Switzerland — Ostreoidea, Zootaxa 3927 (1), pp. 1-117 : 22

Genus Nanogyra Beurlen, 1958

Type species. Gryphaea nana J. Sowerby, 1822 , p. 114, pl. 383 (left figure); BM 43340c, Kimmeridge Clay, England.

Diagnosis (modified from Stenzel 1971, p. N1121). Thin-shelled, inequivalve. Outline suborbicular, subtrigonal, elliptical, or ovate to comma-shaped (virguliform of authors); always opisthogyrate, degree of spirality variable. Left valve moderately convex to globular, smooth or with antimarginal riblets, rarely with small plicae; RV overall slightly concave or convex, with sharp, curved anterior carina. Posterior ligament bourrelet reduced in length, usually forming a ridge with the dorsal margin during early growth stages. Chomata known since Bathonian [see below, N. (P.) reniformis (Goldfuss, 1833) (b)].

Microstructure largely compact foliate some small chambers may be present close to the umbo and shell margin, complex cross-foliation common. Right valve with outer, coarsely prismatic layer ( Siewert 1972, p. 19; Malchus 1990, p. 109; Malchus & Aberhan 1998, p. 629). Most species do not exceed 3 cm in largest diameter, but for some larger species exceptional specimens may reach a height of up to 7 cm.

Remarks. The present diagnosis deliberately excludes? N. (Nanogyra) auricularis (Goldfuss, 1833) (b) as described in Malchus & Aberhan (1998). The species has an unusual large size (ca. 10 cm), numerous large chambers, and a tendency to become uncoiled reaching a ligament area height of up to 4 cm. It is equally unclear whether the specimens from Europe and those from Chile belong to the same species. Malchus & Aberhan (1998) hypothesized that it represents a different genus.