Gryphaea is an extinct oyster; it is also a very distinctive one.
As a bivalve it posseses two valves. The valves are markedly unequal in size and shape, the left valve is strongly incurved, the right valve is small and flat. The left shell is thick and its surface is marked with numerous ridges.
The devil’s toenail found on the Dorset coast is Gryphaea obliquata, it can be distinguished from Gryphaea arcuata as the umbones (beak) of the left valve are more closely incurled in the latter. Gryphaea obliquata is medium sized typically 60mm in length.
They lived on the sea floor with the flat right valve facing up, it probably acted like a lid, when open allowing water carrying oxygen and nutrients to flow in and be filtered out of the water.