Orthoceras are primitive marine cephalopods that possess a shell.
They were most abundant during M.Ordovician period 400 million years ago.

the shell of Orthoceras is shaped like a slowly tapering cylindrical cone and is

made up of closely spaced concavo-convex chambers, joined together by a
centrally placed tube called the siphuncle.

Orthoceras was an active swimmer, with the shell positioned horizontally
in the water.It scavenged and preyed on small animals.