The Collaborative International Dictionary of English v.0.48:

Tridacna \Tri*dac"na\, n. [L., pl., a kind of oysters, fr. Gr. ?
   eaten at three bites, ? tri- + ? to bite.] (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.
   [1913 Webster] Tridactyl

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):

Tridacna gigas
    n 1: a large clam inhabiting reefs in the southern Pacific and
         weighing up to 500 pounds [syn: giant clam, Tridacna
         gigas]