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.

Source: Webster's Revised Unabridged Dictionary (1913)