which are involute, meaning that the larger and more recent whorls overlap and obscure older whorls. The shells of fossil nautiloids may be either straight (i.e., orthoconic as in Orthoceras and Rayonnoceras), curved (as in Cyrtoceras) coiled (as in Cenoceras), or rarely a helical coil (as in Lorieroceras). Some species' shells—especially in the late Paleozoic and early Mesozoic—are ornamented with spines and ribs, but most have a smooth shell. The shells are formed of aragonite, although the cameral deposits may consist of primary calcite. The coloration of the shell of the modern nautilus is quite prominent, and, although somewhat rarely, the shell coloration has been known to be preserved in fossil nautiloids. They often show color patterns only on the dorsal side, suggesting that the living animals swam horizontally. Modern nautiloids Much of what is known about the extinct nautiloids is based on what we know about modern nautiluses, such as the chambered nautilus, which is found in the southwest Pacific Ocean from Samoa to the Philippines, and in the Indian Ocean off the coast of Australia. It is not usually found in waters less than 100 meters (328 feet) deep and may be found as far down as . Nautili are free swimming animals that possess a head with two simple lens-free eyes and arms (or tentacles). They have a smooth shell over a large body chamber, which is divided into subchambers filled with an inert gas (similar to the composition of atmospheric air, but with more nitrogen and less oxygen) making the animal neutrally buoyant in the water. As many as 90 tentacles are arranged in two circles around the mouth. The animal is predatory, and has jaws which are horny and beak-like, allowing it to feed on crustaceans. Empty nautilus shells may drift a considerable distance and have been reported from Japan, India and Africa. Undoubtedy the same applies to the shells of fossil nautiloids, the gas inside the shell keeping it buoyant for some time after the animal's death, allowing the empty shell to be carried some distance from where the animal lived before finally sinking to the seafloor. Nautili propel themselves by jet propulsion, expelling water from an elongated funnel called the hyponome, which can be pointed in different directions to control their movement. Unlike the belemnites and other cephalopods, modern nautili do not have an ink sac, and there is no evidence to suggest