The Arcida is an

extant Extant 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 * Extant taxon, a taxon which is not extinct, ...

order Order, ORDER or Orders may refer to: * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of d ...

bivalve Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, bival ...

mollusc Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is esti ...

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. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start ...

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 The adductor muscles of the hip are a group of muscles mostly used for bringing the thighs together (called adduction). Structure The adductor group is made up of: *Adductor brevis *Adductor longus *Adductor magnus * Adductor minimus This is o ...

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. Ark clams vary both in shape and size. They number about 200 species worldwide. The shells of ark clams are of ...

s or ark shells in the

family Family (from la, familia) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its ...

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 of large and medium-sized marine bivalve mollusks. It includes five families, among them the Pteriidae (pearl oysters and winged oysters). 2010 taxonomy In 2010, a new proposed classification system for the Bivalvia ...

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 basal ...

. 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 (ocean), marine bivalve molluscs related to the ark clams. The genus is the only member of the Family (biology), family Cucullaeidae. Species The World Register of Marine Species l ...

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

Glycymerididae Glycymerididae, often misspelled as Glycymeridae, common names dog cockles or bittersweets, is a worldwide family of salt water clams, marine bivalve mollusks in the order Arcida. They are related to the ark clams. This family contains 45 extan ...

Dall, 1908 (1847) **Family

Noetiidae Noetiidae is a family of saltwater clams, marine bivalve molluscs in the order Arcida. They are related to the ark clams and used to be classified as among them. They are differentiated from the ark clams by the presence of striations on the hin ...

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

External links

{{Taxonbar, from=Q29877127 Bivalve orders