Lamellibranchia \La*mel`li*bran"chi*a\, Lamellibranchiata \La*mel`li*bran`chi*a"ta\, n. pl. [NL. See lamella, and Branchia, Branchiate.]

(Zo["o]l.) A class of Mollusca including all those that have bivalve shells, as the clams, oysters, mussels, etc.

Note: 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.

Source: Webster's Revised Unabridged Dictionary (1913)