Unionida

order of bivalves Unionida (Unionida) is an order comprising closely related families of organisms. It contains 3 families with 52 known species. This includes the family groups Hyriidae, Margaritiferidae, and Unionidae. Among its members, 14 species are currently classified as threatened (Critically Endangered, Endangered, or Vulnerable). It belongs to Bivalvia (Bivalvia).