Bivalvia, Pelecypoda, and Lamellibranchia are synonymous terms for the
same class of molluscs (Bivalvia being the currently accepted one, the
other two now superceded).  The name Bivalvia, as an official taxonomic
designation, includes only bivalved molluscs.  However, the common
english word "bivalves", while usually used to refer to bivalved
molluscs, could in a broader sense include the brachiopods and
ostracods, since these two non-molluscan groups do possess a bivalved shell.

Paul M.