''Adnatoceras'' is a genus of mid Devonian to Pennsylvanian orthoconic nautiloid cephalopods included in the pseudorthocerid family

Spyroceratidae Spyroceratidae are defined Sweet, Walter C. 1964. Nautiloidea -Orthocerida. Treatise on Invertebrate Paleontology, Part K. Geological Soc. of America and Univ of Kansas press as Pseudorthocerida with uniformly slender siphuncle The siphuncle is ...

, characterized by tubular, straight-sided,

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 ...

segments that expand abruptly at the septal foremen. In some species the siphuncle segments become more ovoid in the later grow stages. The name ''Adnatoceas'' comes from the wide area of contact, or adnation, between the connecting ring and flared out brim of the septal neck.

References

Prehistoric nautiloid genera Middle Devonian first appearances Pennsylvanian extinctions {{paleo-nautiloidea-stub