• Synopsis Fresh-Water Moi,i,usca 59 



appearing- toward the disk and not continuous with the sculpture of the 

 latter. Hinge well developed, with strong pseudocardinals, a very strong 



Fig- 175 



interdental projection in the left valve and well developed, strong, but 

 rather short laterals. 



Outer gills only marsupial. 



Type : Arkansia wheelcri O. and W., fig. 175. 



Genus LASMIGONA Rafinesque, 1831. 



Shell elliptic-rhomboid, compressed ; beaks low, their sculpture consist- 

 ing of strong bars ; one pseudocardinal in the right valve and two in the 

 left, the hinder somewhat A -shaped, cutting ofif the hinge-plate in the right 

 valve ; laterals generally imperfect. Marsupium thick, padlike, filling the 

 outer gills. 



Key to the subgenera of Lasmigona. 



{Shell corrugated on posterior slope. .^ Lasmigona s. s. 

 Shell smooth - 2. 



(Hinge teeth delicate 3. 

 Hinge teeth very heavy Pterosyua. 



{ Lateral teeth compressed, moderately developed Platynaias. 



3- 



[ Lateral teeth nearly or quite wanting Alasminota. 



