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Orthoceras ("straight horn") is a genus of extinct nautiloid cephalopod. This genus is sometimes called Orthoceratites.

Orthoceras fossils are common and have a global distribution, occurring in any marine rock, especially limestones.

These are slender, elongate shells with the middle of the body chamber transversely constricted, and a subcentral orthochoanitic siphuncle. The surface is ornamented by a network of fine lirae (Sweet 1964:K224). Many other very similar species are included under the genus Michelinoceras. Although it has an external, chambered shell like the Nautilus, Orthoceras is probably more closely related to coleoids.

The genus was extremely long lived lasting from the Ordovician to the Triassic.

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