Ostreina

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Ostreina (oysters; 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.