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Rhynchorthoceras
Rhynchorthoceras is a Middle Ordovician genus characterized by a rapidly expanded, weakly annulate orthocone, like the orthoconic section of Ancistroceras, but with only a curved, cyrtoconic apex instead of juvenile whorls. Rhynchorthoceras is probably derived from Ancistroceras by a loss of the tarphycerid type apex, although it has been included with the orthocerid Sinoceratidae. References
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