p.11

Exogyra Say
E. costata, Say. Am. Jour. Sc. vol. 2 p. 43-Morton, Synopsis p. 53 pl. 6 figs 1, 2, 3, & 4

Suboval, thick; lower valve convex, costated, transversely corrugated; costoe of the disc somewhat dichotomous, sometimes fornicated; apex lateral, with about two volutions; a single profound cicatrix; hinge with two nearly parallel, deeply excavated grooves, of which the inner one is shortest & corrugated; upper valve flat, with numerous elevated, concentric, squamous plates; outer edge abruptly reflected from the inferior to the superior surface; hinge with a single groove on the edge.

L. 1.0 to 10 inches.