Hinge pliodont, that is to say, it has numerous teeth on either side of the umbones and the teeth are perpendicular to the edge.
Solenopsidae.-Shell equivalve, greatly elongated, umbones very far forward.
Philobryidae.-Shell thin, very inequilateral, anterior part atrophied, umbones projecting.
A, b, right and left valves of the shell; c, d, the umbones or short arms of the lever; e, f, the long arms of the lever; g, the hinge; h, the ligament; i, the adductor muscle.
Prasinidae.-Shell inequilateral, with anterior umbones and prominent anterior auricula; cardinal border arched..
Chamidae.-Shell with sub-equal valves and prominent umbones more or less spirally coiled; ligament external.