Ammonites (AKA snakestones) are quite common around Whitby in the north of England where folklore has it that they are the remains of snakes turned to stone by St. Hilda, the 7th century abbess of Whitby:
The fact that Whitby snakestones generally do not have a head is supposedly due to a convenient beheading curse issued by another Christian martyr, St Cuthbert. However, to make snakestones more saleable, and to reinforce the legend of their origin, serpents’ heads were sometimes carved onto them, especially in Victorian times. (From a collection of ammonite folklore on the British Natural History Museum’s web site).
Via Proceedings of the Athanasius Kircher Society