File:Aviculopecten winchelli (fossil scallop) (Byer Sandstone, Lower Mississippian; Newark, Ohio, USA).jpg

English: Aviculopecten winchelli Meek, 1875 - fossil scallop (partially covered in matrix) in sandstone from the Mississippian of Ohio, USA.

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.

Scallops are common marine bivalves in much of the fossil record and in the modern oceans. Individual bivalve shells, unlike brachiopod shells, are asymmetrical. Scallop shells sometimes approach bilateral symmetry, but the subtriangular, wing-like auricles along the hingeline will still display asymmetry. The fossil scallop seen here is a limonite-stained mold in fossiliferous sandstone. The surrounding fossils include crinoid stem columnals and bryozoans.

Classification: Animalia, Mollusca, Bivalvia, Pteriomorphia, Pterioida, Aviculopectinidae

Stratigraphy: Byer Sandstone Member, lower Logan Formation, upper Waverly Group, Osagean Stage/Series, upper Lower Mississippian

Locality: unspecified site attributed to Newark, central Licking County, east-central Ohio, USA