-Calcareous shells, (usually aragonite), situated laterally around soft body.
-Valves usually symmetrical and of equal size. Usual range of 5-100mm, with very occasional examples of up to 1500mm.
-The valves are opened and closed by muscles which allows filter feeding.
-Classification is based on the number of teeth within the hinge, with the earliest forms having many teeth, progressively reducing in number with evolution.
-Common and diverse fossils found throughout strata.
-There isn't a perceptible diversification in the Mesozoic forms from those found in the Palaeozoic after the decline.
-Triassic "newcomers" may have originated in the Permian just before the P-T boundary.
