Nautiluses, or nautili, are cephalopod molluscs in the family Nautilidae. There are seven known genera of nautilus, all but two of which are extinct; the earliest fossils come from about 230 million years ago, during the mid-Triassic period. Those that remain, in the genera Nautilus and Allonautilus, are the last surviving nautiloids. A total of six species have been described. They are commonly called “living fossils” because they are similar in appearance to many of their prehistoric ancestors. Many people confuse nautili with ammonites, because both are shelled cephalopods, but they are actually two separate groups.