[Gk. nautilos, sailor, or shell-fish supposed to have a membrane which served as a sail.] A genus of shell-fish having a spiral shell, chambered with simple partitions perforated in the centre, concave towards the outlet of the shell. The outer chamber is the largest, and contains the body of the animal. The head of the animal has many simple tapered arms or tentacles. Four kinds are found living in the tropical Pacific, but there are many fossil kinds. It creeps along the bottom of the sea, and does not sail on the surface, as was formerly supposed.