Shell trapezoidal or rounded, with the posterior side longer; ligament usually multivincular; hinge typically taxodont, with the teeth in two similar series, meeting below the umbos, and approximately vertical to the margin of the valve. Animal having a stout, short, deeply grooved foot, very often with a byssus; two adductor muscles of about equal strength; the borders of the mantle bear compound pallial eyes; the labial palps are direct continuations of the lips.
Distribution: All seas, at all depths.