Anthoceras

Anthoceras is a genus of straight, annulated, proterocamerioceratid molluscs from the Lower Ordovician, found in North America, North-Western Australia, and Siberia. The cross section is circular, the siphuncle moderately large, and marginal. Segments are constricted ; septal necks hemichoantici to subholochoantic ; 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 apertural ends are unknown.