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Nuculoida (class Bivalvia, subclass Palaeotaxodonta) An order of bivalves which have a taxodont hinge and equivalve, aragonitic shells. They are isomyarian, the ligament generally amphidetic, and the gills are protobranchiate. The foot is grooved. Adults lack a byssus. Nuculoids are small, infaunal, labial-palp feeders, which first appeared in the Ordovician.