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