This is a 5.9" Nautilus of the genus Cymatoceras from Ambatalafia, Mahajanga Province, Madagascar. As a result of natural geologic processes, there is some warping and damage to the shell that occurred during fossilization.
The shell of a Nautilus is made up of two layers. The inner layer has the iridescent shine while the outer layer provides protection from external forces. Inside the shell are septa which divide the living area of the shell into compartments and help the animal maintain balance and position in the water column.
Nautilus are "living fossils" in that species similar to this one still exist in our oceans.