piddock

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Any of several marine bivalve mollusks of the family Pholadidae, having a long rasping shell used to bore into wood, rock, and clay, and often damaging jetties and wharves.

from The Century Dictionary.

• noun A mollusk of the genus Pholas or family Pholadidæ; especially, a name of those species which are found in British waters, used rarely for food but much for bait, as P. dactylus; a pholad.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Zoöl.) Any species of Pholas; a pholad. See pholas.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Any of the bivalve molluscs of the genus Pholas or family Pholadidae, which burrow into soft rocks.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun marine bivalve that bores into rock or clay or wood by means of saw-like shells