500 unioninje. 



Sub-gen. complanaria, Swainson. 



Shell oblong; the valves connate, the surface plicate. Car- 

 dinal teeth, two or three ; lateral teeth represented by irregular 

 grooves. 



arcula, Lea. marginata, Say. 



complanata, Barnes. rugosa, Barnes. 



confragosa, Say. 



Sub-gen. calceola, Swainson (Uniopsis, Swains.). 



Shell ovate; surface of valves smooth. Cardinal teeth two, 

 irregular or receding from the anterior margin, or with tubercles 

 and undulations representing cardinal teeth. 



calceola, Lea. Raveneliana, Lea. 



radiata, Lea. 



Genus MONOCONDYL$:a, D'Orbigny. 



Shell equivalve, inequilateral, sub-rotund or angulated, 

 covered with a dark epidermis. Hinge composed of a 

 large, obtuse, round cardinal tooth in each valve; lateral 

 teeth none. 



Syn. ? Aplodon, Sjpiw. Monodontina, Conr. 



Ex. M. Guarayana, D'Orbigny, pi. 117, fig. 3. Shell, 

 M. Paraguayana, D'Orbigny, fig. 3, a, 3, b. 



All the species of this genus known are from the rivers of 

 South America. 



