Philobryidae.-Shell thin, very inequilateral, anterior part atrophied, umbones projecting.
Cyrtodontidae.-Extinct; shell equivalve and inequilateral, short, convex.
Lyrodesmidae.-Extinct; shell inequilateral, posterior side shorter; hinge short, teeth in form of a fan.
Mytilidae.-Shell inequilateral, anterior end short; hinge without teeth; ligament external.
Pernidae.-Shell very inequilateral; ligament subdivided; mantle open throughout; anterior adductor absent.
Aviculidae.-Shell very inequilateral; cardinal border straight with two auriculae, the posterior the longer.
Prasinidae.-Shell inequilateral, with anterior umbones and prominent anterior auricula; cardinal border arched..
Megalodontidae.-Shell 1a, tr, Upper and lower inequilateral, thick; posterior siphons adductor impression on a myo ms, Siphonal muscle of the phorous apophysis.
