Orthoceras

Orthoceras ("straight horn") is a genus of nautiloid cephalopod restricted to Middle Ordovician-aged marine limestones of the Baltic States and Sweden.[1] This genus is sometimes called Orthoceratites. Note it is sometimes misspelled as Orthocera, Orthocerus or Orthoceros (Sweet 1964:K222).

Orthoceras was formerly thought to have had a worldwide distribution due to the genus' use as a wastebasket taxon for numerous species of conical-shelled nautiloids throughout the Paleozoic and Triassic. Now, Orthoceras sensu stricto refers to O. regulare, of Ordovician-aged Baltic Sea limestones of Sweden and neighboring areas.[1]

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.