Orthoceras was a marine cephalopod, this particular species was straight shelled and they thrived some 400+ million years ago in the Paleozoic era. Orthoceras is a genus of extinct Nautiloid Cephalopod. These fossils are common and have a global distribution which occur in any marine rock, especially Limestone. These are slender, elongated shells with the middle of the body chamber transversely constricted and a sub-central orthochoanitic siphuncle. The surface is ornamented by a network of fine lirae or fine lines/ridges.