The Collaborative International Dictionary of English v.0.48:

Piddock \Pid"dock\, n. [Etymol. uncertain.] (Zool.)
   Any species of Pholas; a pholad. See Pholas.
   [1913 Webster]

WordNet (r) 3.0 (2006):

piddock
    n 1: marine bivalve that bores into rock or clay or wood by
         means of saw-like shells