The oyster Gryphaea is one of the commonest fossils found in the British Jurassic. The calcite shell is thick and survives well after weathering and erosion of the clays and shales in which it is fossilized. It is also sufficiently resilient to have endured transportation by rivers and Ice Age glaciers - eroded specimens of Gryphaea are often found in river gravels and glacially-deposited boulder clays in regions of England, such as Suffolk and Gloucestershire.
The robust, curved left valve of Gryphaea , marked with prominent growth bands, superficially resembles a thick toenail. It is unclear whether Gryphaea shells were once believed to be the actual toenails of devils, or just that they corresponded with the popular conception of what a devil's toenail ought to look like.
Devil's toenails are particularly common in the Lower Jurassic rocks around Scunthorpe, formerly quarried intensively for the iron ore that was economically important for this Lincolnshire town. They feature in the town's coat of arms, adopted in 1936 (Knell 1988). Knell quoted a passage from a diary written on 10 April 1696 by a local man, Abraham de la Pryne, which says that powdered Gryphaea was used to cure '...a horse's sore back...'
In Scotland, fossil Gryphaea shells are known as clach crubain , translated as 'crouching shell' (Oakley 1974). They were apparently used in the seventeenth and eighteenth centuries to cure pain in the joints. Oakley made the interesting point that their contorted appearance is suggestive of painful joints, an example of sympathetic medicine.