Bivalvia is a taxonomic class
Class (biology)
In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...

 of marine and freshwater molluscs
The Mollusca , common name molluscs or mollusksSpelled mollusks in the USA, see reasons given in Rosenberg's ; for the spelling mollusc see the reasons given by , is a large phylum of invertebrate animals. There are around 85,000 recognized extant species of molluscs. Mollusca is the largest...

. This class includes clam
The word "clam" can be applied to freshwater mussels, and other freshwater bivalves, as well as marine bivalves.In the United States, "clam" can be used in several different ways: one, as a general term covering all bivalve molluscs...

s, oyster
The word oyster is used as a common name for a number of distinct groups of bivalve molluscs which live in marine or brackish habitats. The valves are highly calcified....

s, mussel
The common name mussel is used for members of several families of clams or bivalvia mollusca, from saltwater and freshwater habitats. These groups have in common a shell whose outline is elongated and asymmetrical compared with other edible clams, which are often more or less rounded or oval.The...

s, scallop
A scallop is a marine bivalve mollusk of the family Pectinidae. Scallops are a cosmopolitan family, found in all of the world's oceans. Many scallops are highly prized as a food source...

s, and many other families of molluscs that have two hinged shells. The class was known for some time as Pelecypoda, which is a reference to the soft parts of the animal, whereas the name Bivalvia simply describes the shell, which has two valves. Other names which have been used for this class include Lamellibranchia (referring to the plate-like gill elements, see ctenidium
Ctenidium (mollusc)
A ctenidium is a respiratory organ or gill which is found in many mollusks. This structure exists in bivalves and in many aquatic gastropods, i.e. in some freshwater snails and sea snails and also in some sea slugs...

), Acephala (they have no head), and Bivalva (two valves).

The total number of bivalve species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 is currently approximately 9,200.