the spirally formed mollusc with a ribbed shell,

This is a replica piece.

Ammonites were extremely common in the seas of the Jurassic and Cretaceous periods. They became extinct at the end of the Cretaceous period, about 70 million years ago. The "Nautilus" of the Western Pacific is their closest surviving relative of the "cephalopod molluscs". They lived in tropical waters of all depths, swimming freely in the water rather than on the sea bed and some species grew very large. They are predominantly found in limestone in such places as The Atlas mountains in Morocco and the Dorset coast in the United Kingdom. In some parts of the UK their fossils are known as "snake stones" - the residents of Whitby in Yorkshire have it that St. Hilda miraculously turned snakes to stone explaining local discoveries of these fossils.