The Collaborative International Dictionary of English v.0.48:

Lamellibranchia \La*mel`li*bran"chi*a\, Lamellibranchiata
\La*mel`li*bran`chi*a"ta\, n. pl. [NL. See lamella, and
   Branchia, Branchiate.] (Zool.)
   An earlier name for the class of Mollusca including all those
   that have bivalve shells, as the clams, oysters, mussels,
   etc., now called Pelecypoda or Bivalvia.
   [1913 Webster +PJC]

   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.
         [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Acephala \A*ceph"a*la\, n. pl. [NL., fr. Gr. ?, adj. neut. pl.,
   headless. See Acephal.] (Zool.)
   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.
   [1913 Webster]