The Collaborative International Dictionary of English v.0.48:

Pholas \Pho"las\, n.; pl. Pholades. [NL., fr. Gr. fwla`s,
   fwla`dos, a kind of mollusk.] (Zool.)
   Any one of numerous species of marine bivalve mollusks of the
   genus Pholas, or family Pholadid[ae]. They bore holes for
   themselves in clay, peat, and soft rocks.
   [1913 Webster]