Source : Webster's Revised Unabridged Dictionary (1913)

Quahog \Qua"hog\, Quahaug \Qua"haug\, n. [Abbrev. fr.
   Narragansett Indian poqua[^u]hock.] (Zo["o]l.)
   An American market clam ({Venus mercenaria}). It is sold in
   large quantities, and is highly valued as food. Called also
   {round clam}, and {hard clam}.

   Note: The name is also applied to other allied species, as
         {Venus Mortoni} of the Gulf of Mexico.

Source : WordNet®

quahog
     n 1: Atlantic coast round clams with hard shells; large clams
          usually used for chowders or other clam dishes [syn: {quahaug},
           {hard-shell clam}, {round clam}]
     2: an edible American clam [syn: {quahaug}, {hard-shell clam},
        {round clam}, {Venus mercenaria}, {Mercenaria mercenaria}]