Unionidae is a family of freshwater mussels distributed world-wide, but most diverse in North America with about 297 recognized taxa. Freshwater mussels occupy a wide range of habitats, but most often occupy lotic waters. The shell is equivalve and and has a thick periostracum. The hinge mostly has two cardinals and two posterior lamellar teeth. Unionidae burrow into the substrate, with their posterior margins exposed. They pump water through the incurrent aperture, obtaining oxygen and food. U… (More on Unionidae)