The Collaborative International Dictionary of English v.0.48:

Sea clam \Sea" clam`\ (Zool.)
   Any one of the large bivalve mollusks found on the open
   seacoast, especially those of the family Mactridae, as the
   common American species. (Mactra solidissima or Spisula
   solidissima); -- called also beach clam, and surf clam.
   [1913 Webster]