Dimyaria

Dimyaria, the group of Lamellibranchs including all those bivalve shells the valves of which are closed by a pair of muscles. It includes the two orders Isomya (q.v.) and Heteromya (q.v.); in the former the two muscles are approximately equal, as in the Venus shells, the Areas, etc.; in the latter, the anterior adductor is much the smaller, as in the mussels.