486 Illinois State Lahoratori/ of Natural History. 



twenty-five per cent, of the food of our entire collection, the 

 crustaceans amounted to only fourteen per cent. These divide 

 conveniently into crayfishes, the medium-sized, sessile-eyed 

 crustaceans (Isopoda and Amphipoda), and Entomostraca. 

 The so-called fresh-water shrimps (Palasmon and Palsemonetes) 

 appeared so rarely in the food that they need scarcely be taken 

 into the account. 



Crayfishes made about a sixth of the food of the burbot; 

 about a tenth that of the common perch, a fourth that of half 

 a dozen gars, not far from a third that of the black bass, * the 

 dog-fish, and our four rock bass. Young crayfishes appeared 

 quite frequently in some of the larger minnows (Semotilus and 

 Hybopsis), and also in catfishes, especially the pond and river 

 bull-heads, averaging nearly fifteen per cent, of the entire 

 food of the two most abundant species. 



The small, sessile-eyed crustaceans eaten by fishes were 

 nearly all of four species; viz., Allorchestes dentata, — exces- 

 sively abundant in the northern part of the State, — a species 

 of Gammarus not uncommon in running streams, and two 

 representatives of the isopod genera Asellus and Mancasellus. 

 To fishes at large, this group is of little importance; but the 

 perch of northern Illinois finds about one third of its food 

 among them, and the common sunfishes (Lepomis) eat a con- 

 siderable ratio (eleven per cent,). The miller's thumb of 

 southern Illinois seems also to search for them among the 

 stones. 



The little Allorchestes mentioned above I found in a single 

 white bass, in eleven of the common perch, in one of the largest 

 darters, in five young black bass, in seventeen sunfishes of 

 various species, in the rock bass, the pirate perch, a single 

 grass pickerel and six top minnows, in only two of the true 

 minnow family, in two only of the sucker tribe, in seventeen 

 catfishes, — mostly young or of the smallest species, — in a 

 single dog-fish, and in a single spoon-bill.' The common 



* Our specimens — especially of the small-mouthed black bass — 

 were too few in number to make this average reliable. 

 1 Polyodon. 



