Orthoceras are an extinct species of cephalopod mollusks, that evolved into the squid and octopus of today.  These soft-bodied invertebrates were protected by a hard, straight, conical, chambered shell and were fast-moving carnivores. They moved through the water by squirting water through a siphon, a kind of jet-propulsion, and would catch prey with their tentacles and then give a poison bite with their beak-like jaws.  Orthoceras ranged in size from less than an inch to about 9 feet long.  These nautiliod, sea creatures swam in shallow seas during the mid-Ordovician to the Devonian Periods, about 470 to 360 million years ago.  Today, these fossils are quarried out of limestone and given a high polish and used as bookends, plaques, sculptures and in jewelry.

The name Orthoceras meant "straight horn" and referred to all nautiliods with straight shells.