Palaeoheterodonta is a subterclass of bivalve molluscs. It contains the extant orders Unionida (freshwater mussels) and Trigoniida. 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.

2010 Taxonomy of the Palaeoheterodonta

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. Subterclass: Palaeoheterodonta

Order: Trigoniida

Order: Unionida