File:Cucullaea melhaseana (fossil bivalve) (Pee Dee Formation, Upper Cretaceous; Myrtle Beach, South Carolina, USA) 1.jpg

English: Cucullaea (Idonearca) melhaseana Anderson, 1958 - fossil bivalve from the Cretaceous of South Carolina, USA. (ventral view)

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.

Seen here is an internal mold of a Cucullaea bivalve. The clam's original calcareous shell has dissolved away, leaving an impression of the interior structure of the valves.

Classification: Animalia, Mollusca, Bivalvia, Pteriomorphia, Arcoida, Cucullaeidae

Stratigraphy: Pee Dee Formation, Campanian to Maastrichtian Stage, upper Upper Cretaceous

Locality: unrecorded/undisclosed site at or near Myrtle Beach, coastal-northeastern South Carolina, USA