AMEiuRUS — bullheads; hoened pout 191 



which is never marbled or equally pigmented on both membranes and rays 

 as in A. nehulosus. 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, usually 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 17 to 20 rays, including rudi- 

 ments, usually 18 or 19, its base from 3.9 to 4.4 in length of body, the free 

 margin distinctly rounded; anal rays rather stout, those about middle of fin 

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

 in A. nehulosus, usually 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 rarely (in adults) with 5 to 10 indistinct weak and short teeth; 

 humeral process rather short, rugose, and bluntly pointed. This species 

 much resembles A. nehulosus, and is quite variable, but may usually be 

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

 pectoral spines. 



This, the common bullhead of the IlHnois boy, abundant 

 everywhere in our smaller streams, is distributed throughout 

 the entire length and breadth of the state. In the main features 

 of its distribution 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 yellow bullheads, they show an observable tendency to a 

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

 the species in detail, 247 of the black bullhead and 122 of the 

 yellow. These represent 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 



* Recurring to our detailed collection records, bringing into comparison as to frequency 

 of associate occurrence natalis and nehulosus on the one hand and natalis and melas on 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 second pair (2.12). Since the species 

 of the first pair differ widely in the kinds of Vv-ater bodies which they principally inhabit and 

 those of the second pair agree closely in this respect, we see in the distribution of these species 



