AMEIURUS — bullheads; horned pout 191 



quite evident in the anal fin, which is never marbled or equally pigmented 

 on both membranes and rays as in ,4. nebulosus. Head heavy, 3 . 1 to 

 3 . 5 in length, rather short and much broadened behind, its greatest 

 breadth 3 . 5 to 4 in length of body, usually about 3.7, and sometimes 

 almost equaling its length, in which the width is contained 1.1 to 1 .2, 

 usuallv less than 1.2; mouth wide, the snout short and broadly rounded; 

 jaws about equal; maxillary barbels reaching to or only slightly beyond 

 opercular opening, seldom to tip of humeral process. Dorsal spine variable, 

 in typical specimens rather short and robust and quite or nearly straight, 

 but sometimes long and slender and considerably curved. Caudal slightly 

 emarginate. Anal fin short, of 1 7 to 20 rays, including rudiments, usually 

 18 or 19, its base from 3 . 9 to 4 . 4 in length of body, the free margin dis- 

 tinctlv rounded; anal rays rather stout, those about middle of fin split 

 almost half way to base. Pectoral spine rather shorter and blunter than 

 in .4. nebulosus, usuallv not much curved, its length 2.4 to 3.6 in head, 

 usually a little less than 3 ; the posterior edge usually entire or only slightly 

 roughened, or, more rarelv (in adults) with S to 10 indistinct weak and 

 short teeth; humeral process rather short, rugose, and bluntly pointed. 

 This species much resembles A. nebulosus, and is quite variable, but may 

 usuallv be distinguished by its smaller size, shorter and deeper anal fin, 

 and shorter pectoral spines. 



This, the common bullhead of the Illinois boy, abundant every- 

 where in our smaller streams, is distributed throughout the entire 

 Length and breadth of the state. In the main features of its dis- 

 tribution it agrees with the yellow bullhead, being, like that species, 

 decidedly the most abundant in creeks, and least so in the larger 

 rivers. Its frequency coefficients for our 247 collections containing 

 it are as follows, in the order of their size : creeks, 2.25; the smaller 

 rivers, 1.26; lowland lakes, 1 . 00 ; glacial lakes, .55; the larger rivers, 

 .47. This species also shows a notable preference for the more 

 quiet and muddier parts of the streams it inhabits, as shown by our 

 ratios of 1 . 58 for a muddy bottom and 2.37 for stagnant water or a 

 quiet current. Notwithstanding the similar ecological distribution 

 of the black and yelli rw 1 mllheads, they show an observable tendency 

 to a local separation, as illustrated by a study of our collections of 

 tlic species in detail, 247 of the black bullhead and 122 of the yellow. 

 These represenl 319 separate collections of fishes, only 50 of which 

 contain examples of the two species together, the less numerous 

 species, natalis, occurring in 72 of these collections without the more 

 numerous one.* With reference to the different sections of the 



♦Recurring to our detailed collection records, bringing into comparison as to 

 frequency of associate occurrence natalis and nebulosus on the one hand, and natalis 

 and melas <m the other, and computing the coefficients of association for each of 

 these two pairs of species, we get for the first pair a larger coefficient (3.07) than 



for the se 1 pair (2.12). Since the spei ies of the lirsi pair differ widely in the 



kinds of water bodies which they principally inhabit, and those oi the second pair 



