The nautilus is the only shelled cephalopod. It's regarded as a "living fossil" (I ordinarily do not like the term, but find it appropriate here), as hundreds of millions of years ago they were more varied and far more prosperous. Today there are only six species of nautilus, all limited to the Indo-Pacific.
Description of its shell
Nautiluses are the sole cephalopods whose bony structure of the body is externalized as a shell. The animal can withdraw completely into its shell, closing the opening with a leathery hood formed from two specially folded tentacles.
The nautilus shell presents one of the finest natural examples of a logarithmic spiral. (It is sometimes incorrectly claimed to be a golden spiral as well).
