This is a 7.6" wide cut and polished Cleoniceras ammonite fossil from Madagascar. Ammonites from this location are unusually well-preserved, and are often filled with calcite crystals making it possible to cut them in half and polish them. The details of the spiral chambers remain, and make each one a beautiful object in its own right. It exhibits gorgeous mineralization within the chambers, each separated by white calcite septa (divisions). The chambers are filled with a gorgeous, honey/orange colored agate. It's been sliced in half and polished to reveal the inner chamber detail.
Ammoniteswere predatory mollusks that resembled a squid with a shell. These cephalopods had eyes, tentacles, and spiral shells. They are more closely related to a living octopus, though the shells resemble that of a nautilus. Ammonites appeared in the fossil record about 240 million years. The last lineages disappeared 65 million years ago at the end of the Cretaceous.