Anthoceras is a straight, annulated, proterocamerioceratid

Proterocameroceratidae

The Proterocameroceratidae were the first of the Endocerida. They began early in the Ordovician with Proendoceras or similar genus which had developed endocones, replacing the diaphragms of the ellesmerocerid ancestor....

 (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 half way to almost to the previous septum); connecting rings thick. Endocones are long and slightly asymmetric.

This genus is based on the phargmocone, the chambered part of the shell; the apical and apetural ends are unknown

References

• Curt Teichert, 1964. Endoceratoidea. Treatise on Invertebrate Paleontology, Part K. Geol Soc. of America and Univ of Kansas press. Teichert and Moore (eds)
• Teichert and Glenister 1954 . Early Ordovician cephalopod fauna from northwestern Australia. Bulletins of American Paleontology 35 (150): 7–112.

See also

• Harry Mutvei, 1997. Siphuncular structure in Ordovician endocerid cephalopods. Acta Palaeontologica Polonica vol 42(3) pp375–390 http://www.app.pan.pl/article/item/app42-375.html
• R.E. Crick & Teichert, C. 1983. Ordovician endocerid genus Anthoceras: its occurrence and morphology. Alcheringa: an Australasian Journal of Palaeontology; Vp; 7(2) pp 155–162.