Old Nick's toenails.

I do so wish that Old Nick would sweep up his toenail clippings and not leave them lying around in our bathroom.

Fear not, Devil's toenails are, in reality, the fossilised remains of Gryphaea arcuata an ancient and extinct type of oyster that flourished in our seas some 200 million years ago. The thick, curved shells are reminiscent of a badly manicured "Devil's toenail", hence their common name.