; class Bivalvia, order Pterioda)
A suborder of epifaunal, cemented or free-living bivalves in which the adults are non-byssate, and the foot is absent. The shell is composed of foliated calcite with an aragonitic area below the adductor muscle. In most cases oysters live cemented to a hard substrate by the left valve, in a pleurothetic attitude. They have a monomyarian musculature; the ligament is alivincular and divided into three parts, the central section being the resilium. The hinge margin is edentulous. The gills are eulamellibranchiate, and the pallial line lacks a sinus. Oysters first appeared in the Late Triassic.
Subjects: Zoology and Animal Sciences.