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The soft parts of a typical bivalve are enclosed between two almost identical valves. The insides of the valves are usually pearly. The outsides of the valves have curved ridges. The edge farthest from the hinge, the ventral margin, is often crinkled (crenulate).
Recognising fossil bivalves
Bivalves have two parts (valves) to their shell. The valves are fastened together with a fibre hinge that naturally causes the valves to spring apart. During their lives, the bivalves have a single muscle that keeps the valves closed. When they die, the muscle withers and the springy fibre hinge pops open, so that sea water can get into the valves, and they soon fall apart. For this reason it is rare to find both valves of a fossil bivalve (the example below is an exception).
Brachiopods are the only other two-valved fossil. When brachiopods die, the valves stay closed, and so both valves are found together. This is often the easiest way to distinguish a bivalve from a brachiopod. But, in addition, whereas the brachiopods valves are symmetrical about a line running from front to back, the bivalves are not. Instead, many bivalves have left and right valves that are mirror images.
(Above) Complete bivalve found in mudstone, Jurassic.
(Below) Positions of life of bivalves.
Mytilus (mussel). Tertiary to recent
Attached to rocks by threads called byssus
Ostrea (oyster). Cretaceous to recent
Attached by cement to rocks
Mya. Tertiary to recent
Orthonota (similar to razor clam) Ordovician to Devonian
Long siphons. Deep burrowing
Swims by rapidly shutting valves
Crassatella Cretaceous to recent
Moves using foot
(Below and right) Nuculana
Not symmetrical across both ventral and dorsal valves
Pecten (scallop). Silurian to recent
Symmetrical between valves from dorsal margin