176 FISHES OF ILLINOIS 



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 bull- 

 heads 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 bull- 

 heads, so like that the species can be distinguished with difficulty, 

 diminish mutual competition by difference of ecological preference, 

 and a consequent 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 distribution. The three more 

 abundant stonecats also plainly evade each other, Noturus flavus 

 and Schilheodes miurus by a different general distribution within the 

 state, and both of these avoiding 5. gyrinus by a difference of eco- 

 logical preference, 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, preference, 

 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 SILURIDiE 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 fin complete ; tail deeply forked . . Ictalurus. 



CO. 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 Lcptops; 



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. 



