What is "palaeoheterodonta"

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.