50 



Bryant Walker 



cavity of the beaks not deep ; front part of the shell very heavy, thinner 

 behind. 



Onter gills only serving as marsupium. 



o sinp 



Fig- 157 

 Type: Unio ccsopus Green, fig. 156. Animal, fig. 157. 



Genus PLEUROBEMA ( Rafinesque, 1820) Agassiz. 



Fig. 158 



Fig- 159 



Shell solid, triangular to rhomboid, usually with a prominent umbonal 

 region; beaks at or near the anterior end of the shell, incurved and pointed 

 forward over a small, but well developed lunule ; beak sculpture coarse, con- 

 sisting of a few, often broken, ridges, which curve upward posteriorly ; pos- 

 terior ridge present, but low and rounded ; epidermis showing the rest 

 periods plainly, tawny to olive, often ornamented with rays Avhich show a 

 tendency to break into square spots ; hinge rather strohg, plate generally 

 narrow ; pseudocardinals double in both valves. Cavity of the beaks shal- 

 low. 



Outer gills only serving as marsupium. 



Type: Unio clava Lam., fig. 158. Animal, fig. 159. 



