Orthoceras ("straight horn") is a genus of extinct nautiloid cephalopod. This genus is sometimes called Orthoceratites. Note it is sometimes misspelled as Orthocera, Orthocerus or Orthoceros (:K222). Orthoceras fossils are common and have a global distribution, occurring in any marine rock, especially in limestone. 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 (:K224). Many other very similar species are included under the genus Michelinoceras.