Source : Webster's Revised Unabridged Dictionary (1913)

Tridacna \Tri*dac"na\, n. [L., pl., a kind of oysters, fr. Gr. ?
   eaten at three bites, ? tri- + ? to bite.] (Zo["o]l.)
   A genus of very large marine bivalve shells found on the
   coral reefs of the Indian and Pacific Oceans. One species
   ({T. gigas}) often weighs four or five hundred pounds, and is
   sometimes used for baptismal fonts. Called also {paw shell},
   and {fountain shell}.