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

WordNet (r) 3.0 (2006):

quahog
    n 1: Atlantic coast round clams with hard shells; large clams
         usually used for chowders or other clam dishes [syn:
         quahaug, quahog, hard-shell clam, round clam]
    2: an edible American clam; the heavy shells were used as money
       by some American Indians [syn: quahog, quahaug, hard-
       shell clam, hard clam, round clam, Venus mercenaria,
       Mercenaria mercenaria]