''Calocyrtoceras'' 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 (Cephalopoda, Nautiloidea), from the middle Silurian of North America (Quebec) and central Europe. The shell, or conch, of ''Calocyrtoceras'' is generally characteristic of its family, 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 ...

, an annulated cyrtocone. In the case of ''Calocyrtoceras'' striated both transversely and longitudinally. Similar ''

Cyrtocycloceras ''Cyrtocycloceras'' is a genus of orthocerids from the Middle Silurian of Europe included in the Paraphragmitidae. The shell, or conch, of ''Cyrtocycloceras'' is a moderately expanding annulated exogastric cyrtocone, with curvature like that of ...

'' from the Middle Silurian of Europe is only striated transversely while similar '' Gaspocyclocas'' from the Middle Silurian of North America is only striated longitudinally.

References

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