Class of Mollusca with freshwater, brackish and marine forms with greatly reduced head (no eyes, tentacles or radula) and a laterally compressed body being bilaterally symmetrical. Shell composed of two hinged (lateral) valves, under which two large gills are present for gas exchange and generally for filter feeding. Sexes nearly always separate: trochosphere and veliger larvae. There are fixed forms like mussels and clams and burrowers like shipworm and razor shell. See also Molluscs.

Alternative form for bivalves : bivalvia.