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.
Geisonoceras has an orthoconic shell with transverse lirae and striae that are periodically thickened.