‖n. [ NL. See Trigon. So called in allusion to the triangular shape of some species. ] (Zool.) 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. [ 1913 Webster ]

4. (Zool.) The cutting region of the crown of an upper molar, usually the anterior part. That of a lower molar is the [ Webster 1913 Suppl. ]