Here is an ammonite, probably Graphoceras, with part of the shell removed to reveal the chambers inside. The outer shell has largely broken away and only a small portion remains (the dark brown section towards the centre). The rest of the central part of the ammonite is preserved in creamy-brown coloured calcite, while the outer whorl has been cleaned to show the individual chambers divided by the chamber walls.
This is not the whole ammonite; the last chamber, the long body chamber, is almost completely missing. The last chamber wall is there and if you look carefully, you can see that there is space for another one, but it is not there because this is the start of the body chamber. The body chamber would have wrapped around for at least another half whorl. Part of the outer shell remains in the very bottom of the image.
Find out more about ammonites on the molluscs pages.