(phylum Mollusca, class Bivalvia)
A subclass of equivalve bivalves which have prismatic-nacreous, aragonitic shells, and a small number of teeth that diverge from the umbo. The external ligament is opisthodetic or amphidetic, and parivincular. The hinge plate is present in some forms, and the musculature is variable. The subclass first appeared in the Middle Cambrian, and contains one extinct and two extant orders.
Subjects: Zoology and Animal Sciences.