Scallop, bivalve mollusk (family Pectinidae) distinguished by a shell whose valves are rounded, with a series of ribs radiating across the surface in relief. Scallops are used for food. Unique among bivalves, scallops swim extremely well, propelled by jets of water expelled in snapping the shell shut. There are about 300 species. Chief among the commercial species is the common bay scallop (Argopecten irradians), found in North America.