Cryptodonta is a 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 bivalves. It contains a single extant order, Solemyoida

Solemyoida

Solemyoida is an order of bivalve molluscs.-Families in the order Solemyoida:* Manzanellidae Chronic, 1952 * Solemyidae J. E. Gray, 1840-References:...

, while the Praecardioida are known only from fossil

Fossil

Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...

s.

The valves of the shell are relatively thin and somewhat elongated. Unlike most other bivalves, species in this group have no hinge teeth on their shells. They have relatively primitive, "protobranchiate", 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.

Taxonomy

• Cryptodonta
  • Order †Praecardioida
    • 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...
      
       †Butovicellidae
      • Genus
        Genus
        In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
        
         †Butovicella
    • Family †Praecardiidae
      • Genus †Slava
      • Genus †Cardiola
        Cardiola
        Cardiola is an extinct genus of bivalve mollusk that lived from the Silurian to the Middle Devonian in Africa, Europe, and North America.-External links:* in the Paleobiology Database...
        
        
    • Family †Antipleuridae
      • Genus †Dualina
      • Genus †Hercynella
        Hercynella
        Hercynella is a genus of fossil bivalves of late Silurian or Early Devonian age, found in Europe, North America, western Asia, North Africa and Australia. The name was also invalidly applied to a genus of moths now known as Tulaya....
        
        
  • Order Solemyoida
    Solemyoida
    Solemyoida is an order of bivalve molluscs.-Families in the order Solemyoida:* Manzanellidae Chronic, 1952 * Solemyidae J. E. Gray, 1840-References:...
    
    
  • Superfamily Solemyoidea
    • Family Solemyidae
      Solemyidae
      Solemyidae is a family of protobranch bivalves in the order Solemyoida. .Solemyids are remarkable in that their digestive tract is either extremely small or non-existent, and their feeding appendages are too short to reach outside the shell ....
      
       (Awning clams)
      • Genus Acharax
      • Genus Solemya
    • Family Manzanellidae
      Manzanellidae
      Manzanellidae is a family of bivalves, in the order solemyoida.-Genera and species in the family Manzanellidae:* Huxleyia A. Adams, 1860** Huxleyia munita - minute nucinellid* Nucinella Wood, 1851...
      
      
      • Genus Huxleyia
      • Genus Nucinella