Pteriomorphia is a mollusc subclass

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 the Bivalvia

Bivalvia

Bivalvia is a taxonomic class of marine and freshwater molluscs. This class includes clams, oysters, mussels, scallops, and many other families of molluscs that have two hinged shells...

. Apart from the orders Arcoida

Arcoida

The Arcoida is an extant order of bivalve molluscs. This order dates back to the lower Ordovician period. They are distinguished from related groups, such as the mussels, by having a straight hinge to the shells, and the adductor muscles being of equal size....

, Mytiloida, Ostreoida

Ostreoida

The order Ostreoida includes the true oysters and a number of other related families of bivalves.About eleven families are recognised within it, but suborders, superfamilies and subfamilies are used relatively heavily in this order. The following classification represents a synthesis of different...

, and Pterioida

Pterioida

Pterioida is an order of large and medium-sized saltwater clams, marine bivalve mollusks. It includes five families, among them the Pteriidae -Families within the order Pterioida:* Isognomonidae* Malleidae* Pinnidae...

, it also contains some extinct and probably basal

Basal (phylogenetics)

In phylogenetics, a basal clade is the earliest clade to branch in a larger clade; it appears at the base of a cladogram.A basal group forms an outgroup to the rest of the clade, such as in the following example:...

 families

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...

, such as the Evyanidae, Colpomyidae, Bakevelliidae, Cassianellidae and Lithiotidae.

Pteriomorpha molluscs have lamellibranch gill

Gill

A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water, afterward excreting carbon dioxide. The gills of some species such as hermit crabs have adapted to allow respiration on land provided they are kept moist...

s and are epibenthic. Some can attach to substrate using a byssus

Byssus

Byssus means both a silky filament by which certain molluscs attach themselves to hard surfaces, and a rare fabric, also called sea silk and its fibre source.-Word:...

. One foot is reduced. The mantle

Mantle (mollusc)

The mantle is a significant part of the anatomy of molluscs: it is the dorsal body wall which covers the visceral mass and usually protrudes in the form of flaps well beyond the visceral mass itself.In many, but by no means all, species of molluscs, the epidermis of the mantle secretes...

 margins are not fused. Gills are usually large and assist in feeding. This group includes the well known 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, scallop

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, pen shells, and oyster

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.

Orders in the subclass Pteriomorphia

• Arcoida
  Arcoida
  The Arcoida is an extant order of bivalve molluscs. This order dates back to the lower Ordovician period. They are distinguished from related groups, such as the mussels, by having a straight hinge to the shells, and the adductor muscles being of equal size....
  
   (ark shells)
• Cyrtodontoida†
• Limoida (file shells and allies)
• Mytiloida (saltwater mussels)
• Ostreoida
  Ostreoida
  The order Ostreoida includes the true oysters and a number of other related families of bivalves.About eleven families are recognised within it, but suborders, superfamilies and subfamilies are used relatively heavily in this order. The following classification represents a synthesis of different...
  
   (oysters and scallops)
• Praecardioida†
• Pterioida
  Pterioida
  Pterioida is an order of large and medium-sized saltwater clams, marine bivalve mollusks. It includes five families, among them the Pteriidae -Families within the order Pterioida:* Isognomonidae* Malleidae* Pinnidae...
  
  (winged oysters and allies)