''Cyrtocycloceras'' is a genus of

orthocerid Orthocerida, from Ancient Greek ὀρθός (''orthós''), meaning "straight", and κέρας (''kéras''), meaning "horn", also known as the Michelinocerida, is an order of extinct orthoceratoid cephalopods that lived from the Early Ordovician ...

s from the Middle Silurian of Europe included in the

Paraphragmitidae Paraphragmitidae is an extinct family (biology), family of actively mobile aquatic animal, aquatic carnivorous cephalopods belonging to the Class (biology), subclass Orthoceratoidea endemic to what would be Asia and Europe during the Silurian liv ...

. 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'' 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