A series of Cambrian morphologically intermediate forms show that the bivalved shell of pelecypods is probably derived from the pseudobivalved shell of rostroconchs, which in turn is probably derived from the univalved shell of helcionellacean monoplacophorans. In Ordovician time, pelecypods underwent their first major radiation, which produced seven major groups herein called subclasses. The subclasses Orthonotia and Lucinata are newly named herein; the subclass Actinodontia is elevated to this rank from the rank of order. The group Heterodonta may be polyphyletic, as the taxa included within it seem to be derived from various Ordovician groups, herein regarded as subclasses. It seems likely that the solemyids are descendants of Ordovician palaeotaxodonts. The duplivincular ligament may have had a polyphyletic origin, having evolved independently in the Arcacea, Myalinidae, and Pteriacea.