Trigonia

Trigonia, a genus of bivalved mollusca or Lamellibranciata (q.v.), which is the type of the family Trigoniidae. The shell is trigonal in shape, with a short rounded anterior side, and long, obliquely truncated posterior side. The latter is flattened on the upper margin into a specially ornamented "area," part of which is marked off into an "escutcheon." The genus commenced in the Lias, and is most characteristic of the Oolitic rocks. In the Cretaceous species are still abundant, but they are rare in the Tertiary. The only living forms occur in the Australian seas.