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.
Fuente: Flickr / ametxa