sea clam

from 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]