Synopsis Frksh-Water Mollusca 

 Genus UNIOMERUS Conrad, 1853. 



53 



Shell trapezoidal, with a rounded posterior ridge and pointed or feebly 

 biangulate behind; beaks not prominent, sculptured with curved, rather 



Fig. 164 



strong, concentric ridges; epidermis generally rayless; pseudocardinals us- 

 ually compressed, laterals delicate, slightly curved. 

 Outer gills only marsupial. 



Fig. 165 

 Type: Unio tetralasmus Say, fig. 164. Animal, fig. 165. 



Genus LASTENA Rafinesque, 1820. 



Shell elongated, subsolid, inequilateral, generally wider in front, rounded 

 and truncate at the anterior base, pointed at the post-basal region, and hav- 



Fig. 166 



ing a low posterior ridge, with one or more secondary ridges above it; 

 beaks low, sculptured with a few coarse, irregular, longitudinal folds; epi- 



