The Collaborative International Dictionary of English v.0.48:

Quahog \Qua"hog\, Quahaug \Qua"haug\, n. [Abbrev. fr.
   Narragansett Indian poqua[^u]hock.] (Zool.)
   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.
   [1913 Webster]

   Note: The name is also applied to other allied species, as
         Venus Mortoni of the Gulf of Mexico.
         [1913 Webster] Quaigh