I'm Mike Hermolle, I've been working at the Lapworth Museum as a volunteer for the past four years and I came across this specimen of ammonite. Ammonites are normally used for stratification. They help to identify the ages of the rocks because they are widespread, evolve fairly rapidly and are easily preserved.
This particular specimen, Liparoceras (meaning ”fathead” because of its extreme width) cheltiense, comes from a quarry at Blockley in Gloucestershire and is special because of the state of preservation. Although most of the specimen is lithified and is buried in the matrix, some of the shell remains and shows the internal structure of the animal. In particular the body chamber walls; the scepter are well preserved and you can see how they become more extensive as they approach the edge of the shell, the surface of which can be seen on this specimen, suggesting that this improves the strength of the shell.