A nautilus (from Latin nautilus 'sails like a vessel'; from Ancient Greek ναυτίλος (nautílos) 'seaman, sailor') is any of the various species within the cephalopod family Nautilidae. This is the sole extant family of the infraorder Nautilaceae and the suborder Nautilina.

It comprises nine living species in two genera, the type of which is the genus Nautilus. Though it more specifically refers to the species Nautilus pompilius, the name chambered nautilus is also used for any of the Nautilidae. All are protected under CITES Appendix II. Depending on species, adult shell diameter is between 10 and 25 cm (4 and 10 inches).

The Nautilidae, both extant and extinct, are characterized by involute or more or less convoluted shells that are generally smooth, with compressed or depressed whorl sections, straight to sinuous sutures, and a tubular, generally central siphuncle. Having survived relatively unchanged for hundreds of millions of years, nautiluses represent the only living members of the subclass Nautiloidea, and are often considered "living fossils".

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