From 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
From WordNet (r) 3.0 (2006) :
Tridacna
n 1: type genus of the family Tridacnidae: giant clams [syn:
Tridacna, genus Tridacna]