Source : Webster's Revised Unabridged Dictionary (1913)

Trigonia \Tri*go"ni*a\, n. [NL. See {Trigon}. So called in
   allusion to the triangular shape of some species.] (Zo["o]l.)
   A genus of pearly bivalve shells, numerous extinct species of
   which are characteristic of the Mesozoic rocks. A few living
   species exist on the coast of Australia.