176 FISHES OF ILLINOIS 



heaviest of the fishes of our great rivers. The former lurk, like 

 darters, under stones in small streams, and the latter spend their 

 time in the deeper waters of the Mississippi and the Illinois. 

 The species of Ictalurus prefer clear water and a strong current, 

 while certain of the bullheads thrive in stagnant pools, exposed 

 to the vicissitudes of the overflow and retreat of the waters 

 upon the river bottoms, and liable, indeed, to destruction by 

 the complete drying out of the ponds in which they often become 

 imprisoned. If they succeed in living there, however, until 

 the next overflow, they add by so much to the average catfish 

 population of the streams. Even these bullheads, so like that 

 the species can be distinguished with difficulty, diminish mutual 

 competition by difference of ecological preference, and a con- 

 sequent different local distribution. The yellow and black 

 bullheads, for example, are commonest in creeks (frequencies, 

 2.22 and 2.25), and the brown bullhead in lakes and ponds 

 (frequency, 1.36); and the first two, notwithstanding their 

 similar situations, have been taken together by us less frequently 

 than either of the other two pairs, indicating some difference of 

 local preference within the limits of their like more general dis- 

 tribution. The three more abundant stonecats also plainly 

 evade each other, Noturus flavus and Schilbeodes miurus by a 

 ' different general distribution within the state, and both of 

 these avoiding S. gyrinus by a difference of ecological prefer- 

 ence, being most abundant in clear swift waters, while gyrinus 

 is found most frequently in quiet waters over a mud bottom. 

 By all these various characteristics of structure, habit, prefer- 

 ence, and capacity, the family is remarkably adapted to life 

 in our interior waters, and its predominance in them is thus 

 easily understood. 



Key to the Genera of SILURID^ found in Illinois 



a. Adipose fin with its posterior margin free. 



b. Premaxillary band of teeth without lateral backward extensions; anal rays 



17 to 35, including rudiments. 



c. Bony bridge from occiput to dorsal fln complete; tail deeply forked.. .Ictalurus. 

 cc. Bony bridge from occiput to dorsal fin broken; caudal fin typically rounded, 



truncate or slightly emarginate (forked in A. lacustris) Ameiurus. 



bb. Premaxillary band of teeth with a backward extension on each side; anal 



rays 12 to 15, including rudiments Laptops. 



aa. Adipose fin adnate to the back, continuous with the caudal and separated 



from it only by a notch. 



d. Premaxillary band of teeth with lateral backward extensions, as in Leptops ; 



skin thick, tough, and villose, not translucent Noturus. 



dd. Premaxillary band of teeth truncate at the ends, as in Ameiurus; skin thin- 

 ner than in d, smooth or very finely villose, sometimes translucent 



Schilbeodes. 



