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. They formed as grains of sand or shell fragments rolled around in the warm water depositing layer upon layer of calcium in concentric rings.
The chambered shell has been filled with both sediment and calcite. The long body chamber is probably complete as it takes up the last half of the outer whorl. This is where all the soft parts would have been housed: stomach, gills and other gooey bits.
Find out more about ammonites on the molluscs pages.