any of various snails (class Gastropoda, phylum Mollusca) having a flattened shell. Most marine species cling to rocks near shore. A common American species is the Atlantic plate limpet (Acmaea testudinalis) of cold waters; the common species of Britain and northern Europe is Patella vulgata. Keyhole limpets, of the prosobranch family Fissurellidae, have a slit or hole at the apex of the shell, through which water currents (and wastes) exit.