CATFISH GENUS NOTURUS RAFINESQUE 123 



39.48; upper Missouri R., Nebraska to Montana 15 (37-40) 38.93; 

 Smoky Hill R., Kansas 5 (38) 38.00; Neosho R., Kansas 4 (38-40) 

 38.75; Missouri and Mississippi Rivers, Missouri 24 (37-40) 38.50; 

 total 133 (37-41) 39.21. 



Specimens collected from the channels of the Missouri River 

 throughout Missouri and the Mississippi River below the mouth of 

 the Missouri River generally have smaller eyes than specimens from 

 elsewhere. Although a few small eyed individuals have been obtained 

 from the mouths of tributary streams, the small eye size seems to be 

 restricted to the area of these highly turbid big rivers. Specimens 

 from the Mississippi River above the mouth of the Missouri River 

 have normal size eyes. 



The eye size is variable, but the eyes are usually obviously small. 

 Occasional individuals in a sample have relatively large eyes, and in 

 these samples a few appear to have eyes of intermediate size. The eye 

 oiflavus from the upper Mississippi River and other clear streams is 

 usually stepped into the snout length 2.5 to 3.4 times. In 15 of the 

 Missouri and Mississippi River specimens the eye is stepped into 

 the snout 2.5 to 4.7, mean 3.6 times. 



Because the eyes of these specimens show considerable size varia- 

 tion, especially within a small sample, from tiny to almost normal 

 size, and because the specimens appear otherwise typical of flavus 

 I am inclined to believe that the eye size is simply a response to the 

 muddy, turbid river waters. Degeneration or loss of eyes within a few 

 years has been observed in bullhead populations that have been 

 subjected to reduced light or darkness. 



Further study of these or similar specimens should be made to 

 compare the structure of the eye in detail with that of other specimens. 



Aside from the eye size these specimens do not appear to differ 

 from other populations oiflavus in their morphology. Their color is 

 drab, usually medium gray in preservation, but the distribution of 

 pigment does not differ from that of other flavus. 



Distribution. — Noturus flavus (map 8) occurs in the Mississippi 

 River system, in the Mohawk and Hudson River systems, and in the 

 Great Lakes-Saint Lawrence River drainage. 



It has entered the Mohawk and upper Hudson River systems 

 (Greeley and Bishop, 1933, pp. 89, 98; and Greeley, 1935, pp. 86, 

 96) from the Great Lakes, presumably as a canal immigrant. After 

 the recession of Wisconsin ice, several entrances were made from the 

 Mississippi drainage into the Great Lakes basin; one was undoubtedly 

 into the Lake Michigan basin, another into the Lake Erie basin, and 

 one was into the Lake Superior tributaries. In the Lake Michigan 

 basin, flavus is confined to the area south of the Kalamazoo River in 

 Michigan and to the southern half of Wisconsin and southward. 



298-943 O— 69 9 



