An earlier name for the class of Mollusca
including all those that have bivalve shells, as the clams, oysters, mussels, etc., now called Pelecypoda
. [1913 Webster
" 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. Called also Conchifera, and Pelecypoda. See Bivalve.