The largest species of chambered nautilus:
There’s a whole list of names applied (not always consistently) to this particular part of the animal kingdom, so hang on tight while I attempt to sort things out for you.
Nautilus is the common name for a family of cephalopods, more formally known as the nautilidae. Depending on who you listen to, there are either three or six living species in the family, in two genera — “chambered nautilus” is informally used for any of them, but properly is specific to the species Nautilus pompilius, the largest species in the genus.
In any event, the nautilidae have survived millions of years without much change — so they’re somewhat of a living fossil. They’re the only living cephalopods with an external shell, have simple eyes without lenses (they see with the equivalent of a pinhole lens), and have much simpler brains than do their other living cousins (squid and octopuses). They’re hard to keep and have never been successfully raised in captivity, so you’ll likely only see them alive in certain parts of the Pacific.
This fine specimen was seen at the California Academy of Sciences in Golden Gate State Park, San Francisco.