The fan mussel, Atrina fragilis, is a species of large saltwater clam, a marine bivalve mollusc in the family Pinnidae, the pen shells. The fan mussel is one of the largest (30 to 48 cm long) European bivalve molluscs. It is one of the rarest species of marine mollusc in the United Kingdom - so rare that surveys of nearly 9,000 sites around Britain between 1987 and 1998 found none.