Cyrtoceras is an dubious extinct genus of oncoceridan nautiloids that was used for fossils from the middle Ordovician to the middle Devonian, in Africa, Europe, North America and South America.
As for 1964, this genus is no longer considered as valid, as it has not been correctly established.
References
Sources Fossils (Smithsonian Handbooks) by David Ward Fossils (A Golden Guide from St. Martin's Press) by Frank H. T. Rhodes, Paul R. Shaffer, Herbert S. Zim, and Raymond Perlman Aquagenesis: The Origin and Evolution of Life in the Sea by Richard Ellis
External links Cyrtoceras in the Paleobiology Database