Paleoheterodonta is a subclass of bivalve molluscs. It contains the 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...

 Unionoida

Unionoida

Unionoida is an order of freshwater mussels, aquatic bivalve mollusks. The order includes most of the larger freshwater mussels, including the freshwater pearl mussels...

 (freshwater mussels) and the prehistoric 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

Heterodonta

Heterodonta is a subclass of bivalve molluscs, including the clams and cockles. They are distinguished by having the two halves of the shell be equally sized, and having a few cardinal teeth separated from a number of long lateral teeth. Their shells lack a nacreous layer, and the gills are...

.