Bivalves, also known as Lamellibranchs or pelecypods, are a class of the phylum mollusca (molluscs). Bivalve softparts are completely enclosed within two identical but left-handed and right-handed, hinged calcareous shells. Muscles are used to close the shells but an elastic ligament opens the gape. Bivalves are known from the mid Cambrian. Cockles and mussels are living examples of bivalves.
This picture shows Nucula ovata which lived close to the surface in the Gault Clay sea. It was found in the Cretaceous cliffs of Kent, England.