(genus Crepidula), any marine snail belonging to the family Calyptraeidae (subclass Prosobranchia, class Gastropoda), in which the humped or flattened shell has a decklike half partition inside. Slipper shells occur worldwide in shallow waters. Adults are fixed to rocks or live within the empty shells of other mollusks. The common Atlantic slipper shell (C. fornicata), often called slipper limpet, is about 4 cm (1.5 inches) long and yellowish; it is abundant from Nova Scotia to Texas. In addition, C. fornicata has been introduced to the west coast of the United States, the coastal waters of Asia, and the coastal waters of England, France, and other European countries. In these locations, slipper shells have become a nuisance in oyster beds.