Hinge with schizodont teeth; embryo a glochidium. 
Family Unionidae. 
Subfamily Unioninae. Beak sculpture concentrie; embryos in outer or all four of the gills. (Exobranchiae.) 
Ovisacs distinetly marked out by a sulcus. 
Ovisacs not separated by a sulcus. 
Male and female shells alike. 
(Mesogenae.) 
Marsupium a few ovisacs in the 
center of outer gills. 
(Ptychogenae.) 
Marsupium occupying the whole 
outer gills in a series of folds. 
(Eschatigenae.) 
Marsupium occupying the outer 
border of outer gills. 
(Diagenae.) 
Ovisacs filling the outer gills, 
running, crosswise. 
(Homogenae.). 
Marsupium filling the entire outer 
gills and forming smooth pads. 
(Tetragenae.) 
Marsupium filling all four gills, 
forming smooth pads; bleak cavities 
deep. 
Se Agassiz. 
Obliquaria Rafınesque. 
Ptychobranchus Simpson. 
Dromus Simpson. 
Strophitus Rafinesque. 
Anodonta Bruguiere. 
Colletopterum Bourguignat. 
Gabillotia Servain. 
Leguminaia Conrad. 
Lastena Rafinesque. 
Solenaia Conrad. 
Gonidea Conrad. 
Anodontoides Simpson. 
Pegias Simpson. 
Arcidens Simpson. 
Symphinota Lea. 
Alasmidonta Say. 
Hemilastena Agassiz. 
Margaritana Schumacher, 
Unio Retzius. 
Pleurobema Rafinesque. 
Quadrula Rafinesque. 
Schistodesma Simpson. 
eben! Simpson. 
Cuneopsis Simpson. 
