308 KANSAS UNIVERSITY SCIENCE BULLETIN. 



terior slope sharply truncated, often at right angles to the 

 lateral slope dorsally. Shell concentrically sculptured with 

 smooth, continuous ridges, which are much crowded on the 

 posterior slope. Epidermis from horn color to deep chestnut 

 brown, and more or less marked with a series of curved rays, 

 rays, which are made up of a series of widely separated dark 

 dots. Ligament short, thick, dark brown. 



Interior : Pseudocardinals double in the left and single in 

 right valve, erect, large, pyramidal, coarsely serrate. Lat- 

 erals of moderate length, curved or almost straight, oblique, 

 heavy, tending to be double in both valves. Interdentum 

 large and smooth, but cut away back of the right pseudo- 

 cardinal. Muscle cicatrices of moderate size, the two adduc- 

 tor scars of about equal size and excavation. Pallial line 

 well impressed, often its entire length. Dorsal cicatrices a 

 line of pits in the wall of the umbone cavity. Cavity of the 

 umbones moderately large, of the shell slight, that of the 

 female greater than that of the male shell. Nacre milk white, 

 very slightly iridescent posteriorly. 



This species is distributed generally over the Mississippi 

 drainage as far south as Louisiana, and is also found in the 

 Tombigbee and Alabama river systems. Its distribution in 

 Kansas is limited to the clear-water rivers of the southern 

 drainage. The exact localities from which it has been re- 

 ported are : Spring river, Baxer Springs (Cragin) ; Fall river, 

 Wilson county (Mead and Popenoe) ; Neosho river, at Os- 

 wego (Newlon), and at Burlington and the southern state 

 line. At the last locality it is quite common in the rocky 

 riffles. The species is not confined to riffles, however, but 

 occurs in a variety of locations. 



This form shows a decided variation, which is dependent 

 directly on its habitat. In localities where rocks abound and 

 the current is swift the animal develops a strong musculature 

 and a stout, heavy shell. Slow- flowing streams with muddy 

 beds cause the animal to develop a long, light, greatly com- 



