''Monocyrtoceras'' is a genus of armenoceratids (

Cephalopod A cephalopod is any member of the molluscan class Cephalopoda (Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head ...

a) from the Middle Silurian of North America (Wisconsin), with a gently curved shell and a

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 ...

like that of ''

Elrodoceras ''Elrodoceras'' is a genus of armenoceratids, (Cephalopoda, Actinocerida), that grew fairly large, characterized by a straight shell, slightly curved at the apex, a siphuncle narrower than that of ''Armenoceras'' but with segments still wider th ...

''. According to Teichert, 1964, in the

Treatise A treatise is a formal and systematic written discourse on some subject, generally longer and treating it in greater depth than an essay, and more concerned with investigating or exposing the principles of the subject and its conclusions."Treat ...

part K, ''Monocyrtoceras'' along with ''Elrodoceras'' and ''

Armenoceras ''Armenoceras'' is a genus of actinocerid nautiloid cephalopods whose fossils ranged from the late Whiterockian Stage in the early Middle Ordovician, through the remainder of the period and on into the Upper Silurian. It is the type genus of th ...

'' forms one of the two branches of the

Armenoceratidae The Armenoceratidae are a family of early Paleozoic nautiloid cephalopods belonging to the order Actinocerida.Flower 1957.Studies of the Actinoceratida.; Memoir 2; New Mexico Bureau of Mines and Mineral Resources, Socorro, NM Teichert 1964. Act ...

References

* Curt Teichert, 1964. Actinoceratoidea, Treatise on Invertebrate Paleontology, Part K. Geological Society of America and University of Kansas Press. Prehistoric nautiloid genera Actinocerida {{paleo-nautiloidea-stub