The Arcida is an

extant Extant or Least-concern species, least concern is the opposite of the word extinct. It may refer to: * Extant hereditary titles * Extant literature, surviving literature, such as ''Beowulf'', the oldest extant manuscript written in English * Exta ...

order Order, ORDER or Orders may refer to: * A socio-political or established or existing order, e.g. World order, Ancien Regime, Pax Britannica * Categorization, the process in which ideas and objects are recognized, differentiated, and understood ...

bivalve Bivalvia () or bivalves, in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class (biology), class of aquatic animal, aquatic molluscs (marine and freshwater) that have laterally compressed soft bodies enclosed b ...

mollusc Mollusca is a phylum of protostome, protostomic invertebrate animals, whose members are known as molluscs or mollusks (). Around 76,000 extant taxon, extant species of molluscs are recognized, making it the second-largest animal phylum ...

s. This order dates back to the lower

Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era, and the second of twelve periods of the Phanerozoic Eon (geology), Eon. The Ordovician spans 41.6 million years f ...

period. They are distinguished from related groups, such as the

mussel Mussel () is the common name used for members of several families of bivalve molluscs, from saltwater and Freshwater bivalve, freshwater habitats. These groups have in common a shell whose outline is elongated and asymmetrical compared with other ...

s, by having a straight hinge to the shells, and the adductor muscles being of equal size. The duplivincular ligament, taxodont dentition, and a shell microstructure consisting of the outer crossed lamellar and inner complex crossed lamellar layers are defining characters of this order. Seven families are currently recognised within the order, including the well-known

ark clam Ark clam is the common name for a family of small to large-sized saltwater clams or marine bivalve molluscs in the family Arcidae. Generally less than 80 mm long, ark clams vary both in shape and size. They number about 200 species worldwide. T ...

s or ark shells in the

family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...

Arcidae.

Taxonomy

The order Arcida, as the suborder Arcacea, is included in the order Taxodonta by R.C. Moore, 1952,R.C, Moore, Pelecypods, Ch 10, Inverrtebrate Fossils, Moore, Lalicker, and Fischer. McGraw-Hill, 1952. characterised by simple hinge-line dentition consisting of small, numerous, similar hinge teeth, separate mantle lobes, poorly developed siphons, and filibranch gills. In 2010, Bieler, Carter & Coan proposed a new classification system for the Bivalvia which combines the taxodont Arcida with the dysodont

Limida The Limidae or file shells are members of the only family of bivalve molluscs in the order Limida. The family includes 130 living species, assigned to 10 genera. Widely distributed in all seas from shallow to deep waters, the species are usually ...

Mytilida Mytilida is an order of marine bivalve molluscs, commonly known as true mussels. There is one extant superfamily, the Mytiloidea, with a single extant family, the Mytilidae. Species in the order Mytilida are found worldwide, but they are more ...

Ostreida The order Ostreida includes the true oysters. One superfamily (Ostreoidea) and two extant families are recognised within it. The two families are Ostreidae, the true oysters, and Gryphaeidae The Gryphaeidae, common name the foam oysters or ...

and

Pteriida The Pteriida are an order (biology), order of large and medium-sized Marine (ocean), marine bivalve mollusc, mollusks. It includes five families, among them the Pteriidae (pearl oysters and winged oysters). 2010 taxonomy In 2010, a new propos ...

as the

Pteriomorphia The Pteriomorphia comprise a subclass of saltwater clams, marine bivalve molluscs. It contains several major orders, including the Arcida, Ostreida, Pectinida, Limida, Mytilida, and Pteriida. It also contains some extinct and probably ...

. Subtaxa included in the Arcida are shown below. In 2016, the superfamilies of Arcida changed from two to three, with an additional superfamily of fossils only. The new taxonomy is as follows: *Superfamily Arcoidea Lamarck, 1809 **Family Arcidae Lamarck, 1809 **Family Catamarcaiidae Cope, 2000 **Family

Cucullaeidae The false ark shells (''Cucullaea'') are a small genus of marine bivalve molluscs related to the ark clams. The genus is the only member of the family Cucullaeidae. Species The World Register of Marine Species lists the following species: *†' ...

Stewart, 1930 **Family Frejidae Ratter & Cope, 1998 **Family

Glycymerididae Glycymerididae, often misspelled as Glycymeridae, common names dog cockles or bittersweets, is a worldwide Family (biology), family of salt water clams, Marine (ocean), marine bivalve mollusks in the order Arcida. They are related to the ark clam ...

Dall, 1908 (1847) **Family Noetiidae Stewart, 1930 **Family Parallelodontidae Dall, 1898 *Superfamily Glyptarcoidea Cope, 1996: with fossil species only. **Family Glyptarcidae Cope, 1996 **Family Pucamyidae Sánchez & Benedetto, 2007 *Superfamily Limopsoidea Dall, 1895 **Family Limopsidae Dall, 1895 **Family Philobryidae F. Bernard, 1897

References

{{Taxonbar, from=Q29877127 Bivalve orders