This is a sectioned ammonite from the Inferior Oolite limestone. The ooliths, the tiny egg-shaped structures that make up the Inferior Oolite, can clearly be seen in some of the chambers but others have been filled with finer sediment. The ooliths formed as grains of sand or shell fragments rolled around in the warm water depositing layer upon layer of calcium in concentric rings.
This is not a complete ammonite; you can tell that because there is no long body chamber in the outer whorl. The chamber walls of ammonites are fairly plain when sectioned through the middle of the shell, but they become folded like a curtain towards the outer shell, forming a complex suture line.
Find out more about ammonites on the molluscs pages.