(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, labialpalp feeders, which first appeared in the Ordovician.
Subjects: Zoology and Animal Sciences.