ioo8 



FRESH-WATER BIOLOGY 



141 (146) Male and female shells different; female shell with a decided infla- 

 tion in the post-basal region, which is thinner than the rest 

 of the shell, of different texture, often toothed, and usually 

 radiately sculptured; hinge complete; marsupium occupying 

 the posterior part of the gill only. 



Truncilla Rafinesque . . 142 

 Four subgenera: 142, 143, 144, i45- 



142 (143) Male shell smooth, no radial groove in front of the posterior ridge. 

 Female with a high posterior ridge, posterior slope flattened. 



Subgenus Truncilla s.s. 



The type, T. triquetra Raf. (Fig. 1508), occurs from western New York to Nebraska and south 

 to Kansas and northern Alabama. Five other species are found in Tennessee and Alabama. 



Fig. 1S08. 



143 (144) Male shell with a wide, radiating, shallow depression in front of the 

 posterior ridge. Female with a small, rounded, well-defined 

 radial post-basal swelling. . . Subgenus Scalenaria Agassiz. 



The type, Truncilla sulcata Lea (Fig. 1509), ranges from the Tennessee River north to 

 southern Michigan. Two other species occur in Tennessee and Georgia. 



Fig. 1509. 



