318 KANSAS UNIVERSITY SCIENCE BULLETIN. 



nating with each other on the opposite valves ; posterior to 

 the knobs a very shallow furrow is generally present. Um- 

 boidal ridge permanent, continuous, rounded ; posterior slope 

 slightly excavated, ornamented with small, obscure, transverse 

 ridges. Epidermis smooth ; in color brown, color sometimes 

 arranged in concentric bands of lighter and darker shades, 

 frequently rayed with obscure, narrow, wavy lines of dark 

 green. Lines of growth rounded and continuous, often dark 

 colored. Ligament short, thick, dark brown ; lunule small 

 but well marked. 



Interior : Pseudocardinals large, ragged, pyramidal, double 

 in the left and single in the right valve, the right tooth sur- 

 rounded by a trench in which the left pseudocardinals lock. 

 Laterals low, thick, short, slightly curved. Anterior ad- 

 ductor cicatrix small, well excavated, long and narrow, the 

 floor roughened ; posterior scars rather small, well impressed, 

 distinct. Pallial line impressed for anterior half. Dorsal 

 cicatrices forming a line on the base of the pseudocardinals. 

 Cavity of beaks variable, generally small, of the shell small. 

 Nacre silver white, slightly iridescent posteriorly. 



Dorsal pos- Number of 

 Length. Height. Breadth. Um. ra. terior angle, nodules. 



56 50 32 0.30 132° 4-5 (23.1) 



48 34.5 28.5 .30 131° 4-4 (34.1) 



62 49 36.5 .27+ Curved. 4^ (22.1) 



58 45 27.5 .29 144° 4-3 (22.4) 



42.5 29 18 .26+ Curved. 3- (326.1) 



This species ranges from Michigan south and southwest to 

 Alabama and central Texas. In Kansas it is found in the 

 clear-water rivers of the southern drainage and in them is 

 an occasional but not a common species. Only one specimen 

 has been found in the Marais des Cygnes drainage. This 

 was a single valve from the Marais des Cygnes at Ottawa. 

 Cragin similarly reports a single specimen from the Kansas 

 drainage, found at Mill creek, in Wabaunsee county. The 

 favorite habitat is gravel-beds. No species is more distinct 

 than this one. It can be distinguished from any other Kan- 

 sas Unio without difficulty by the prominent lateral knobs 

 alone. There is comparatively little variation. 



