Palaeoheterodonta is a subclass of bivalve molluscs. It contains the extant orders Unionoida (freshwater mussels) and Trigonioida. They are distinguished by having the two halves of the shell be of equal size and shape, but by having the hinge teeth be in a single row, rather than separated into two groups, as they are in the clams and cockles.
In 2010 a new proposed classification system for the Bivalvia was published in by Bieler, Carter & Coan revising the classification of the Bivalvia, including the subclass Paleoheterodonta. Superfamilies and families as listed by Bieler et al. Use of † indicate families and superfamilies that are extinct.
Subclass: Palaeoheterodonta
†Beichuanioidea Liu & Gu, 1988†Beichuaniidae Liu & Gu, 1988†Megatrigonioidea Van Hoepen, 1929†Megatrigoniidae Van Hoepen, 1929†Iotrigoniidae Savelive, 1958†Rutitrigoniidae Van Hoepen, 1929†Myophorelloidea Kobayashi, 1954†Myophorellidae Kobayashi, 1954†Buchotrigoniidae Leanza, 1993</small†Laevitrigoniidae Savelive, 1958†Vaugoniidae Kobayashi, 1954Trigonioidea Lamarck, 1819Trigoniidae Lamarck, 1819†Eoschizodidae Newell & Boyd, 1975 (syn: Curtonotidae)†Groeberellidae Pérez, Reyes, & Danborenea 1995†Myophoriidae Bronn, 1849 (syn: Cytherodontidae, Costatoriidae, Gruenewaldiidae)†Prosogyrotrigoniidae Kobayashi, 1954†Scaphellinidae Newell & Ciriacks, 1962†Schizodidae Newell & Boyd, 1975†Sinodoridae Pojeta & Zhang, 1984Superfamily †Archanodontoidea Modell, 1957 (placement in Unionoida uncertain)Family †Archanodontidae Modell, 1957Superfamily Etherioidea Deshayes, 1832Family Etheriidae Deshayes, 1832 (syn: Mulleriidae, Pseudomulleriidae)Family Iridinidae Swainson, 1840 (syn: Mutelidae, Pleiodontidae)Family Mycetopodidae Gray, 1840Superfamily Hyrioidea Swainson, 1840Family Hyriidae Swainson, 1840Superfamily †Trigonioidoidea Cox, 1952Family †Trigonioididae Cox, 1952Family †Jilinoconchidae Ma, 1989 (placement uncertain)Family †Nakamuranaiadidae Guo, 1981 (syn:Sinonaiinae, Nippononaiidae)Family †Plicatounionidae Chen, 1988Family †Pseudohyriidae Kobayashi, 1968Family †Sainschandiidae Kolesnikov, 1977Superfamily Unionoidea Rafinesque, 1820Family Unionidae Rafinesque, 1820Family Liaoningiidae Yu & Dong, 1993 (placement uncertain)Family Margaritiferidae Henderson, 1929 (syn:Margaritaninae, Cumberlandiinae, Promargaritiferidae)Family †Sancticarolitidae Simone & Mezzalira, 1997
