Anthoceras

Anthoceras is a genus of straight, annulated, proterocamerioceratids (Order Endocerida) from the Lower Ordovician, found in North America, NW Australia, and Siberia. The cross section is circular, the siphuncle moderately large, and marginal. Segments are constricted (producing concave profiles in internal molds); septal necks hemichoantici to subholochoantic (reaching halfway to almost to the previous septum); connecting rings thick. Endocones are long and slightly asymmetric.
This genus is based on the phragmocone, the chambered part of the shell; the apical and apetural ends are unknown.