Characteristic and unmistakable shell even when juvenile one. Subtriangular in form, not much inequilateral but strongly inequivalve. Right valve quite bigger and more convex than the left one, which fits snugly into it. Big and inflated umbo in the right valve which has a concentric striae sculpture. The left valve always showes a persistent periostracum (generally absent in the right valve), brown in colour covering the whole valve made exception for the umbonal zone and it is more extended in the posterior area. Radial zones either reddish or rosy in colour on white-yellowish background colouring are sometimes present on the right valve. Generally the inside of the left valve is more coloured than the right one (violet, pink, brown in colour).

This shell is variable in its form, in its solidity and in the globosity of the valves, besides in the colouring pattern, and this is the reason for several variety in names. The most important one is rosea (Thrown, 1844) former Leach ms. species, found at 1400 m depth. It was considered as a species apart due to a bigger length, a thinner concentric sculpture on the right valve and for a rosy colouring pattern, but really there were found all transition forms and now it is considered a C. gibba synonym. The adult specimens reach an average length of 10 mm.

Common enough all over the Mediterranean and the Black Sea too. Habitat: this shell prefers muddy or sandy-muddy bottoms. It was found up to 2000 m depth.