Orthoceras are an extinct nautiloid cephalopod that used to roam the oceans 400 million years ago and can be found in Sweden, the mountains in Morocco, and even Antartica. They had a long, straight shell unlike its cousins the Nautilus and Ammonite, and it grew as the Orthoceras did- some even as long as 14 feet! They moved in groups so many Orthoceras fossils are found in large clusters, similar to what's seen on this plate.