The Geisonoceras Ammonoids are extinct members of the cephalopod group of mollusks. Only a limited number of species have survived to the present day which includes the squid and the chambered nautilus. Like their younger relative Ammonites, they had a multi-chambered shell, and added more chambers as it grew, always living in the outer chamber. They thrived in the Devonian period some 400 to 350 million years ago.
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.