(Science: zoology) a class of mollusca including all those that have bivalve shells, as the clams, oysters, mussels, etc.
They usually have two (rarely but one) flat, lamelliform gills on each side of the body. They have an imperfectly developed head, concealed within the shell, whence they are called acephala.
Synonym: Conchifera, and Pelecypoda. See bivalve.
Origin: NL. See lamella, and branchia, branchiate.