Characteristic Features

-Thin, elongate or rounded-oval shells

-Inequilateral and mostly inequivalve

-Valves usually slightly gaping

-Posterior end often more or less pointed

-Umbones small and low

-Ventral margin often sinuate

-Smooth or only weakly developed ornament

-Pallial sinus well developed

-Usually thin periostracum

-Widely distributed in most area