E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Favourite tokens worn by pilgrims in their hats. The polished side of the shell was scratched with some rude drawing of the blessed Virgin, the Crucifixion, or some other subject connected with the pilgrimage. Being blessed by the priest, they were considered amulets against spiritual foes, and might be used as drinking vessels.