Orthoceras was an extinct cephalopod related to the modern squid, octopus, and Nautilus. It lived from 500-200 million years ago, and existed in the Earth's oceans for approximately 300 million years. As the Orthoceras specimen grew, a septa (the horizontal line you seen in a number of Orthoceras fossils) grew as a dividing wall to separate the old and new chambers. The siphuncle is the long line down the middle and was a tube that could be filled with air for buoyance or push the air out for jet-propulsion.

Orthoceras fossils are most often embedded in limestone all over the world, and are often found in large groups.