The largest subclass of the class Bivalvia in the phylum Mollusca. Lamellibranchs are ciliary filter feeders with greatly enlarged ctenidia, in each of which the elongated gill filaments are held together in parallel series to form folded lamellae. The 30,000 living species constitute the most successful of molluscan groups, in terms both of individual numbers with their biomass and of ecological bioenergetics (calorie transfer in food chains).
There are several proposed systems of ordinal classification for the subclass Lamellibranchia, but little agreement on the best structural bases. One system frequently employed comprises six orders: Anisomyaria, Taxodonta, Heterodonta, Schizodonta, Anomalodesmata, and Adapedonta. There is more general acceptance of the (possibly 18) superfamilies of lamellibranchs, each with characteristic shell structure, hinge dentition, ligament, and degree of fusion of the mantle lobes involving the origin of the siphons. The more important superfamilies include Mytilacea (mussels), Ostreacea (true oysters), Cardiacea (true cockles), Unionacea and Corbiculacea (fresh-water clams), Myacea (including softshell clams), Mactracea (surf clams), Tellinacea, and Pholadacea (=Adesmacea). Within each larger superfamily there is a wide range of adaptive radiation, with different genera living as shallow or deep burrowers or as sessile attached forms. There can be considerable evolutionary convergence shown by genera from different superfamilies, and deep burrowers with massive fused siphons are found in five superfamilies. See also Bivalvia; Boring bivalves; Mollusca.
