Arcoida
Encyclopedia
The Arcoida is an extant order
Order (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...

 of bivalve molluscs. This order dates back to the lower Ordovician
Ordovician
The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...

 period. They are distinguished from related groups, such as the mussel
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, by having a straight hinge to the shells, and the adductor muscle
Adductor muscle
- Humans :* Adductor muscles of the hip, the most common reference in humans, but may also refer to** Adductor brevis muscle, a muscle in the thigh situated immediately behind the pectineus and adductor longus...

s being of equal size.

Seven families are currently recognised within the order, including the well-known ark clam
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....

s or ark shells in the family
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...

 Arcidae.

Classification

  • Superfamily Arcoidea
    • Arcidae
      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....

    • Cucullaeidae
      False ark shell
      The false ark shells are a small genus of marine bivalve molluscs related to the ark clams. The genus is the only member of the family Cucullaeidae.-Species:* Cucullea labiata * Cucullea concomorata...

    • Noetiidae
      Noetiidae
      Noetiidae is a family of bivalves related to the ark clams. They are differentiated from the ark clams by the presence of striations on the hinge ligament...

    • Parallelodontidae
      Parallelodontidae
      The Parallelodontidae is a family of bivalves, related to the ark clams. It contains at least four genera.-Species:* Grammatodon* Parallelodon** Parallelodon rugosus* Pleurogrammatodon* Porterius** Porterius Dalli...

  • Superfamily Limopsoidea
    • Glycymerididae
      Glycymerididae
      Glycymerididae, previously known as Glycymeridae, common names dog cockles or bittersweets, is a worldwide family of marine bivalve mollusks, related to the ark clams. In this family the shell is generally round in outline, is slightly longer than it is wide, and the external ligament lacks...

    • Limopsidae
      Limopsidae
      Limopsidae is a family of bivalves, related to the ark clams and bittersweets. This family contains about thirty species in seven genera.-Species:* Crenulilimopsis Kuroda & Habe, 1971* Cnisma Mayer, 1868* Empleconia Dall, 1908...

    • Philobryidae
      Philobryidae
      Philobryidae is a family of bivalves, related to the ark clams and bittersweets. Species are found in most seas, in shallow to moderately deep water...

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