Orthoceras "straight horn" is a genus of extinct cephalopod. Orthoceras are dated to the lower Ordovician to Triassic ages, 500 to 190 million years ago. They are ancestors to the modern day squid. The Orthoceras ranged in size from a few inches in length to over six feet. They could swim as well as crawl on the ocean floor. By filling the chambers in their shells with air they could float through the seas propelling themselves by squirting jets of water.