HADROPTERUS — BLACK-SIDED DARTERS 283 



the banks of the Illinois River, and from bays and bottom-land lakes 

 connected with that stream. It is not a swift-water species, and 

 has but little in its habits, food, or favorite situations, to identify 

 it with the darters at large. 



Outside Illinois it occurs in all the Great Lakes, in Lake Cham- 

 plain, in the St. Lawrence River, and in various smaller streams in 

 Quebec, and thence southward to Virginia and the Ohio basin, west- 

 ward to Kansas and Missouri, and southwestward to Alabama and 

 Trinity River in Texas. 



It is sometimes taken on the hook with a worm bait, and it is prob- 

 ably the only one of our darters definitely known as an angler's fish. 



This species is particularly changeable in color, as observed by us 

 in aquarium specimens, the darker tints sometimes deepening to 

 black, and the gold and emerald complexion of the cheeks and 

 opercles becoming extraordinarily bright. It was noticed that the 

 lower part of the transverse bars would sometimes blacken inde- 

 pendently of the upper part, giving an appearance of a row of lateral 

 blotches like those of Hadropterus aspro. 



A third of the food of eleven specimens was found by us to con- 

 sist of crustaceans (mainly Entomostraca) , and the remainder of in- 

 sects, the latter chiefly Chironomiis larvae, larvas of day-flies, and 

 water-bugs (Corixa). 



Genus HADROPTERUS Agassiz 



(black-sided darters) 



Body rather elongate, compressed or not; mouth rather wide, termi- 

 nal; premaxillaries not protractile; teeth on vomer and usually on pala- 

 tines; belly with a median series of enlarged ctenoid plates, which in most 

 species fall off at intervals, but are persistent in some; vertebrae (four 

 species) 39 to 42 (18 or 19 + 20 to 23) ; pyloric caeca 2 to 4. Darters of 

 more or less slender and graceful form, of active habits, and of moder- 

 ately brilliant coloration; size various, some species reaching a length of 

 6 to 8 inches, others much smaller; species 11 or 12. 



Key to the Species of HADROPTERUS found in Illinois 



a. Gill-membranes not broadly united at isthmus, distance from tip of snout to- 



angle formed by their union scarcely exceeding that to back of orbit. 



b. Color pattern transverse, consisting of (1) bars or bands, or (2) of blotches- 



and transversely rather than longitudinally arranged marblings. 



c. Sides with about 15 blotches, some of them extending upward and down- 



ward so as to form ill-defined bars; cheeks scaled evermanni. 



cc. Sides with about 7 broad transverse bars, extending from below lateral line 

 on one side, across back and down on other side; cheeks naked. . . .evides. 



