The Collaborative International Dictionary of English v.0.48:

Lamellibranch \La*mel"li*branch\, n. (Zool.)
   One of the Lamellibranchia (also called Pelecypoda). Also
   used adjectively.
   [1913 Webster] Lamellibranchia

WordNet (r) 3.0 (2006):

lamellibranch
    adj 1: bivalve [syn: lamellibranch, pelecypod,
           pelecypodous]
    n 1: marine or freshwater mollusks having a soft body with
         platelike gills enclosed within two shells hinged together
         [syn: bivalve, pelecypod, lamellibranch]