Bivalve oyster shell
Oyster shells changed through time as the animals evolved. Gryphaea shapes differ in Lower and Upper Jurassic rocks, becoming noticably larger or dilated (hence the species name dilatata).
Oysters are bivalves, of which there are two different kinds. Mussels, scallops and clams have a solid, hard shell, whereas the shell of the oyster is made of many fine layers.
Find out more about bivalves on the molluscs pages.