Nanogyra (Palaeogyra) Mirkamalov, 1963

Koppka, Jens, 2015, Revision of the Bivalvia from the Upper Jurassic Reuchenette Formation, Northwest Switzerland — Ostreoidea, Zootaxa 3927 (1), pp. 1-117 : 29

Subgenus Nanogyra (Palaeogyra) Mirkamalov, 1963

Type species. Ostrea virgula Goldfuss, 1833 (b) [= Nanogyra virgula ( Deshayes, 1831) ], original designation by Mirkamalov (1963, p. 152); Upper Jurassic, Germany.

Remarks. Mirkamalov (1963) introduced Palaeogyra as a genus, unaware of Nanogyra ( Beurlen, 1958) . Subsequently, Stenzel (1971: N1121) regarded Palaeogyra as a junior synonym of Nanogyra . That view does not reflect the morphologic differences (ornamentation, chomata) within Jurassic Exogyrinae which diverged into two distinct lineages during the Middle and Upper Jurassic. Nanogyra (Palaeogyra) is here considered as a subgenus of Nanogyra for species with developed chomata.

Original diagnosis. "Elongated bivalves, frequently curved, thin-shelled, lower shell with radial striations, upper shell smooth, plain. Upper Jurassic of Western Europe and the European part of Russia." ( Mirkamalov 1963, p. 152) (translated herein from Russian).

Emended diagnosis. Inequivalve, outline virguliform or sickle-shaped (occasionally posteroventrally bilobate), oval or triangular; always opisthogyrate; degree of spirality variable but usually moderate; ligament area exogyroid, small and narrow, as for the genus; shell thin; microstructure as for the genus ( Siewert 1972). Left valve flat to moderately convex; smooth or covered with fine antimarginal riblets or anteriorly with few antimarginal plicae; RV flat, nearly smooth, but antimarginals present; anterior carina absent, weak or distinctly developed; chomata always present in both valves. Species usually smaller than 3 cm (diagnosis based on N. virgula , N. catalaunica , N. reniformis , and N. welschi ).