Tri·dac·na n.  Zool. A genus of very large marine bivalve shells found on the coral reefs of the Indian and Pacific Oceans. One species (Tridacna gigas) often weighs four or five hundred pounds, and is sometimes used for baptismal fonts. Called also paw shell, and fountain shell.
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 Clam n.
 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.
    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).
    Clams, or clamps, is a shellfish not much unlike a cockle; it lieth under the sand.   --Wood (1634).
 2.   Ship Carp. Strong pinchers or forceps.
 3. pl. Mech. A kind of vise, usually of wood.
 Blood clam. See under Blood.
 Tridacna gigas
      n : giant clam inhabiting reefs in the southern Pacific weighing
          up to 500 pounds [syn: giant clam]