The figure of this month's MUSSELp Mussel of the Month is from Spix & Wagner's 19th century treatment of the freshwater Mollusca of Brazil. They actually favored two different names for the shell illustrated above: Unio caudatus and Diplodon furcatum! This potential confusion, however, is moot since the same species was described earlier by Schumacher under the name Prisodon obliquus.
                   
                
                  Classification:
                  Phylum Mollusca
                    Class Bivalvia
                    Subclass Palaeoheterodonta
                    Order Unionoida
                  Superfamily Etherioidea Deshayes, 1830
                  Family Hyriidae Swainson, 1840
                    Subfamily Hyriinae s.s.
                    Tribe Hyriini
                  Genus Prisodon Schumacher, 1817
                  Species Prisodon obliquus Schumacher, 1817
                    (+ Unio caudatus + Diplodon furcatum, Spix & Wager, 1827)