Bivalves are bilaterally symmetrical molluscs having two calcareous, asymmetrical shells (valves) - they include the clams, oysters, and scallops. In most bivalves, the two shells are mirror images of each other (the major exception is the oysters). They occur in marine, estuarine, and freshwater environments. Bivalves are also known as pelecypods and lamellibranchiates.

Bivalves are sessile, benthic organisms - they occur on or below substrates. Most of them are filter-feeders, using siphons to bring in water, filter the water for tiny particles of food, then expel the used water. The majority of bivalves are infaunal - they burrow into unlithified sediments. In hard substrate environments, some forms make borings, in which the bivalve lives. Some groups are hard substrate encrusters, using a mineral cement to attach to rocks, shells, or wood.

The fossil record of bivalves is Cambrian to Recent. They are especially common in the post-Paleozoic fossil record.

The angular surf clam shown above is part of the South African Province: "The huge waves and cool waters of South Africa have produced a molluscan fauna dominated along its rocky shores by large limpets and abalones. Its beaches are often strewn with colorful, offshore cones, trochids and volutes. At certain seasons the cast-off egg-cradles of three species of paper nautiluses are found abundantly on some beaches." [info. from museum signage]

Classification: Animalia, Mollusca, Bivalvia, Heterodonta, Veneroida, Mactridae