''Cyrtocycloceras'' is a genus of

orthocerid Orthocerida is an order of extinct Orthoceratoid cephalopods also known as the Michelinocerida that lived from the Early Ordovician () possibly to the Late Triassic (). A fossil found in the Caucasus suggests they may even have survived until ...

s from the Middle Silurian of Europe included in the

Paraphragmitidae Paraphragmitidae is an extinct family of actively mobile aquatic carnivorous cephalopods belonging to the subclass Orthoceratoidea endemic to what would be Asia and Europe during the Silurian living from 436 to 418.7 mya, existing for approxima ...

. The shell, or conch, of ''Cyrtocycloceras'' is a moderately expanding annulated exogastric cyrtocone, with curvature like that of a rocking chair rocker. Annulations, encircling ribs, are close spaced and transverse and the surface is transversely striated. '' Calocyrtoceras'' from the Middle Silurian of both Europe and North America is striated both transversely and longitudinally while ''

Gaspocyrtoceras ''Gaspocyrtoceras'' is a genus of orthocerids included in the Paraphragmitidae Paraphragmitidae is an extinct family of actively mobile aquatic carnivorous cephalopods belonging to the subclass Orthoceratoidea endemic to what would be Asia a ...

'' is striated only longitudinally. Moreover the annulations on ''Calocyrtoceras'' and ''Gaspocyrtoceras'' tend to be thicker and further apart than on ''Cyrtocycloceras''.

References

* Walter C. Sweet, 1964. Nautiloidea- Orthocerida. Treatise on Invertebrate Paleontology, Part K. Geological Society of America. Prehistoric nautiloid genera Silurian animals {{paleo-nautiloidea-stub