Source : Webster's Revised Unabridged Dictionary (1913)

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}.

Acephala \A*ceph"a*la\, n. pl. [NL., fr. Gr. ?, adj. neut. pl.,
   headless. See {Acephal}.] (Zo["o]l.)
   That division of the Mollusca which includes the bivalve
   shells, like the clams and oysters; -- so called because they
   have no evident head. Formerly the group included the
   Tunicata, Brachiopoda, and sometimes the Bryozoa. See
   {Mollusca}.