Orthoceras (“straight horn”) is an extinct squid-like mollusk from the Ordovician period, about 450 million years ago. The creature lived in the last open segment on the wide end of the shell, and ranged from 1 cm to 6 ft long. As the body outgrew the segment, a dividing wall, called the septa, grew to separate the old home segment from the new. A small tube, the siphuncle, runs down the center of the shell and was used for propulsion and buoyancy. These fossils are found in the Atlas Mountains of the Sahara Desert in Morocco.