Why is a Gryphaea dilatata called devils toenail?

The name "devil's toenail" is not commonly used to refer to Gryphaea dilatata. It is more frequently called the "Devil's toenail shell" when referring to a fossil of the Jurassic belemnite species Passaloteuthis bisinuata. The name "devil's toenail" is associated with this species because of the long, pointed guard on the shell that resembles a sharp toenail.