Nautilidae

Nautilidae is the sole living family of the superfamily Nautilaceae, subclass Nautiloidea, within the cephalopod class, represented in the fossil record from the Triassic to the present. The family comprises two genera and nine living species in total: Allonautilus, with two species, and Nautilus, with seven, including the type species Nautilus pompilius. Nautilids have involute to loosely convoluted, generally smooth shells, compressed or depressed in cross-section, built of aragonite with a nacreous inner layer and threaded by a tubular siphuncle running through the chambers. Unlike other living cephalopods, nautilids bear 50 to 90 tentacles rather than eight or ten arms, and their tentacles lack suckers; the group is often called a living fossil for having changed comparatively little over some 500 million years.