Just found it while out shooting near the Coast. It looks like my whippets toenail, but about 500 times bigger!
Fossil Hunter: tinbum
Location: Cornwall, United Kingdom
[Editor's Note: Nice find and you would not be the first to be fooled by one of these fossils! What you are looking at is a piece of the Shell of a fossil Ammonite. Those ridges on the inside are the internal structure of the shell. The southern coast of the UK is well known for its spectacular ammonite fossils notably in the area of Lyme Regis. You will need to fill in the missing pieces of the shell with your imagination, but compare it to some of the other ammonite fragments here by typing "ammonite" in the search box.]