Trigonoceras is an extinct prehistoric nautiloid genus from the nautilid

Nautilida

The Nautilida constitute a large and diverse order of generally coiled nautiloid cephalopods that began in the mid Paleozoic and continues to the present with a single family, the Nautilidae which includes two genera, Nautilus and Allonautilus, with six species...

 family Trigonoceratidae that lived during the Early Carboniferous

Carboniferous

The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...

 in what is now western Europe.

Trigonoceras has a very loosely coiled gyroconic or cyrtoconic shell with a subtriangular cross section, as indicated by fragmentary specimens. The venter on the outer rim or convex side is broad and transversely concave with angular shoulders; the sides which converge on a narrow dorsum are broadly convex. The siphuncle

Siphuncle

The siphuncle is a strand of tissue passing longitudinally through the shell of a cephalopod mollusk. Only cephalopods with chambered shells have siphuncles, such as the extinct ammonites and belemnites, and the living nautiluses, cuttlefish, and Spirula...

is slightly dorsal of the center and is orthochoanitic.(Kummel,B.1964.) A member of the Trigonoceratidae, Trigonoceras is also a component of the superfamily Trigonocerataceae