Crepidula fornicata is a species of medium-sized sea snail, a marine gastropod mollusc in the family Calyptraeidae, the slipper snails and cup and saucer snails. It has many common names, including common slipper shell, common Atlantic slippersnail, boat shell, quarterdeck shell, fornicating slipper snail, Atlantic slipper limpet and it is in Britain as the "common slipper limpet".