Ammonites are an extinct group of marine animals of the subclass Ammonoidea in the class Cephalopoda, phylum Mollusca. They are excellent index fossils, and it is often possible to link the rock layer in which they are found to specific geological time periods.

In medieval Europe, fossilised ammonites were thought to be petrified snakes, and were called "snakestones" or, more commonly in medieval England, "serpentstones". They were taken to be evidence for the actions of saints such as Saint Hilda and Saint Patrick. Traders would occasionally carve the face of a snake into the empty, wide end of the ammonite fossil and sell them to the public. Ammonites from the Gandaki river in Nepal are known as saligrams, and are believed by Hindus to be a concrete manifestation of God or Vishnu.

This piece was cast from an ammonite I found whilst walking along Chesil Beach, Dorset; part of the Jurassic Coast, England.