Gryphaea arcuata is an extinct species of foam oyster, a bivalve mollusc in the family Gryphaeidae from the Early Jurassic of Europe.
It is commonly referred to in English folklore as the 'devils toenail' due to its supposed resemblance to the devil's 'cloven hoof'.
Sources
Gryphaea arcuata in the Paleobiology Database Fossils (Smithsonian Handbooks) by David Ward (Page 101)