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

The Collaborative International Dictionary of English v.0.48:

Clam \Clam\ (kl[a^]m), n. [Cf. Clamp, Clam, v. t.,
   Clammy.]
   1. (Zool.) A bivalve mollusk of many kinds, especially those
      that are edible; as, the long clam (Mya arenaria), the
      quahog or round clam (Venus mercenaria), the sea clam or
      hen clam (Spisula solidissima), and other species of the
      United States. The name is said to have been given
      originally to the Tridacna gigas, a huge East Indian
      bivalve.
      [1913 Webster]

            You shall scarce find any bay or shallow shore, or
            cove of sand, where you may not take many clampes,
            or lobsters, or both, at your pleasure. --Capt. John
                                                  Smith (1616).
      [1913 Webster]

            Clams, or clamps, is a shellfish not much unlike a
            cockle; it lieth under the sand.      --Wood (1634).
      [1913 Webster]

   2. (Ship Carp.) Strong pinchers or forceps.
      [1913 Webster]

   3. pl. (Mech.) A kind of vise, usually of wood.
      [1913 Webster]

   Blood clam. See under Blood.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Venus mercenaria
    n 1: 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]