A living ammonite was kind of like an octopus with a shell. Today they are represented only by fossils.

Sometimes ammonites are split open and the inner surfaces are polished to reveal the chambers inside. Usually both halves are sold as a unit and it makes an educational and very attractive display.

This ammonite is quite a bit larger than the whole specimen which was part of an $8 fossil kit.

Though the most familiar ammonites are the flat spiral type, similar to a ram's horn that gave them their name, there are several other kinds as well.

Some had shells that were nearly straight, others were straight and then coiled then straight again. Some had a helical spiral like a snail, but the strangest shells were those of Nipponites.

Nipponites had a shell that seems to curve at random in all three dimensions. The shape was so bizzare that the first discovery was thought to be a diseased individual. Subsequent disoveries showed that it wasn't a fluke. A close analysis of the curves shows that they are actually formed by a series of bent U shpes. Some have speculated that the changing direction of growth gave the shell neutral buoyancy. This has been largely dismissed. Another theory is that the odd curves made it difficult for a predator to find the open end when the animal retreated into its shell.

Split ammonite

Stereoscopic scanning