For a long time, the classification of Unio was in a state of confusion— both in the sense of which species properly belong to Unio and where that genus goes in the family-level classification of freshwater mussels. Much of the 20th century was spent organizing the Unionoida into new genera, so that helped fix the first problem. The correct position of Unio was addressed in a recent paper by Dan Graf.
Classification:
Phylum Mollusca
Class Bivalvia
Subclass Palaeoheterodonta
Order Unionoida
Superfamily Unionoidea Rafinesque, 1820
Family Unionidae s.s.
Subfamily Unioninae s.s.
Tribe Unionini
Genus Unio Philipsson in Retzius, 1788
Species Unio pictorum (Linnaeus, 1758)
