trigonia

from The Century Dictionary.

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

from the GNU version of the Collaborative International Dictionary of English.

• noun (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.