Cyrena

Cyrena, a genus of bivalved mollusca and the type of the family Cyrenidae: the shell is generally somewhat wedge-shaped. Cyrena is of interest to geologists, as it usually inhabits estuaries and brackish water, and thus its abundance in some Eocene and older rocks indicates their estuarine formation. Corbicula fluminalis is a close ally of Cyrena; it is very common in the gravels of the London area, and survives only in some rivers in the eastern Mediterranean.