Genus CARYOCORBULA Gardner, 1926

Caryocorbula Gardner, 1926: type species Corbula alabamiensis Lea, 1833 (original designation); Eocene of East Coast of USA.

Serracorbula Olsson, 1961: type species Serracorbula tumaca Olsson, 1961 (original designation), = Corbula nasuta G. B. Sowerby I 1833; Recent, tropical East Pacific.

Diagnosis. Shell trigonal to ovate-elongate, small to moderate in size, subequivalve to inequivalve, subequilateral, with a weak to moderate keel between the central and posterior slope. Posterior slope elongated, forming a rostrum. Right valve slightly larger than left, with a marginal groove along the free internal margin where the opposite valve fits. Sculpture of weak to strong commarginal ribs, similar or slightly different on both valves; fine radial striations may also be present. Lunule area not demarcated. Right valve hinge with anterior cardinal tooth upcurved at the dorsal margin, and with a posterior deep resilial socket. Left valve hinge with anterior cardinal socket to insertion of right cardinal tooth, and with posterior chondrophore; chondrophore moderately broad, projected and flattened, with a weak to well developed median ridge on the dorsal side, frequently provided with a narrow to large posterior knob. Nepioconch not distinguished.

Remarks: Caryocorbula includes most American species assigned to Cuneocorbula Cossmann, 1886 (Gardner 1926; Anderson 1996). According to Coan (2002), Serracorbula Olsson, 1961, is a junior synonym of Caryocorbula . It is difficult to define Caryocorbula, as its characteristics are present to a greater or lesser degree in other genera of Corbulinae . The species of Caryocorbula can be discriminated from other Corbulinae by an integrated set of features: trigonal or ovate-elongated shell with a keel between the central and posterior slope and elongated posterior rostrum, a moderately broad and projected chondrophore and the lack of a nepioconch.

According to Goodwin et al. (2008) Caryocorbula species have a GF2 growth pattern without a nepioconch. The species analyzed in the present work seem to agree with this pattern, except for Caryocorbula chittyana which has a nepioconch-like pre-accretion shell.