Synopsis Fresh-Water MolIvUSca 59 



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

 latter. Hinge Avell 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: Arkovsia whcclcri O. and W., fig. 175. 



Genus LASMIGONA Rafinesque, 183 1. 



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 off the hinge-plate in the right 

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

 outer gills. 



Key to the subgenera of Lasinigona. 



{Shell corrugated on posterior slope Losniigoiia s. s. 

 Shell smooth 2. 



r Hinge teeth delicate 3. 



[ Hinge teeth very heavy Ptcrosyiia. 



{ Lateral teeth compressed, moderately developed Platynaias. 



3 



[ Lateral teeth nearl}^ or quite wanting 4lasuiin 



Ota. 



