\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}.