trigonia

Century Dictionary and Cyclopedia

1. n. The typical genus of the family Trigoniidæ. T. margaritacea is the pearly trigon. See also cut under Trigoniidæ.
2. n. A shell of the genus Trigonia or family Trigoniidæ; a trigon: also used attributively: as, the trigonia beds or grits.

GNU Webster's 1913

1. n. (Zoöl.) 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.