It is an ammonite called Baculites, from near the famous Coon Creek fossil site in Tennessee. Ammonites were cephalopods, related to squid, octopus, and the chambered nautilus. Like the nautilus, an ammonite's shell was separated into chambers, but instead of having a simple, straight wall between the chambers, the chamber walls for ammonites were very intricate. Each species had its own pattern, which can be used to identify them. Part of the shell has been eroded away, showing the chamber pattern underneath.