Bivalves form one of the largest and most easily recognisable groups of molluscs. They have two, usually symmetrical, shells attached by a hinge and are extremely common fossils in many places in the British isles. There are about 15,000 species alive today.

Most bivalves are marine and live in shallow water. Many of them burrow in the soft sediment with a foot which emerges from between the valves. Others bore into hard rock, and some attach themselves to objects.

They appeared in the Cambrian period and dwindled in the Permian, then increased in number until their peak Tertiary. They are still numerous today.

See also: gryphaea (a common bivalve)
See also: brachiopods

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