Cameroceras is a species of Ordovician nautiloid that had a straight shell right behind its head. It belongs to a group of nautiloids called orthocones, along with Orthoceras, Endoceras, and Gonioceras.
Although the widely accepted size estimate of Cameroceras's length is 20 feet, there is some debate. Paleontologists often find partial shells of Cameroceras, very rarely the whole thing. When they do find a complete specimen of the shell, it is usually of a small individual. Unless we have the living chamber or the tip of the shell in the specimen, we cannot accurately determine the length of the animal. Based on partial specimens of large individuals, we can only know that it could have grown very big, but not the exact length. In the future we may find a way to determine which part of the shell the fossil belonged to.
In the image below, I drew Cameroceras hunting near the seabed. This individual has successfully caught the eurypterid Megalograptus. Another Megalograptus is swimming away, and Isotelus is crawling on the sea floor directly below the Megalograptus. On the left side, near the head of Cameroceras, there are two rugose corals and one crinoid.
Like the modern nautilus, Cameroceras probably had an extremely strong grip with its tentacles. Once something was caught, it would be very hard for the prey to escape. The tentacles were probably stronger than those of the nautilus, because Cameroceras was much bigger (the modern nautilus only has a shell diameter of 8 inches).
Cameroceras had an amazing way of keeping its head from facing towards the bottom of the ocean and the tip of its shell from facing towards the surface. Cameroceras had a long siphuncle, a kind of tube, running down from its siphon. The siphuncle had traffic cone-shaped blocks of calcium in it, which counter-weighted the body and kept it horizontal. Like all nautiloids, it had upward-facing rings called septa. They were filled with gas and kept Cameroceras afloat. It was a very efficient strategy of locomotion.
The siphon, which was connected to the siphuncle, sucked in water and then shot it out again to propel Cameroceras in the opposite direction of whatever way the extremely flexible siphon was pointing. Modern cephalopods can swim backwards and forwards and also steer very well, because of the flexibility of their siphon. Cameroceras probably had a very flexible siphon too, and this extreme maneuverability would have made it an efficient hunter.
A Sea Without Fish by David L. Meyer and Richard Arnold Davis, pg. 132-134.
Thanks to Paul Mayer at the Field Museum for discussing how paleontologists find out the size of orthocones when they don't have the complete shell.