502 Illinois State Laboratory of Natural History. 



Finally, eight of the slender, active, and wholly predaceous 

 little brook silversides (iy«5?(^es^Aess/cc?*ZMs) had eaten a single 

 fish, fourteen per cent, of Entomostraca, and about eighty per 

 cent, of insects — somewhat more than half of aquatic origin. In 

 brief, the structures of Labidesthes, the habits of Phenacobius 

 and the darters, and the differences in size of the species of 

 Boleosoma an(J Hadropterus were all reflected in the food of this 

 little group. 



The obverse fact of the unifying effect of similarity of 

 alimentary structures is apparently shown by a small collection 

 of minnows, all belonging to the first two groups of the paper 

 cited above*, made from an extremely muddy little creek in 

 Jersey county, which contained no visible vegetation and few, 

 if any, Entomostraca. Twelve of these fishes, representing the 

 genera Campostoma, Pimephales, Hyborhynchus, Hybognathus, 

 and Notemigonus, agreed in food almost precisely, all having 

 swallowed the fine mud of the creek bottom, with a slightly 

 varying admixture of unicellular Algse and vegetable debiis. 



As an example of a contrast between two species agreeing 

 in alimentary structures, but differing in size and somewhat, 

 also, in habitual range, we may take three examples of Notropis 

 lieterodon and three of Notropis inegalops, captured at 

 McHenry, May 8, 1880. More than half the food of the latter 

 group consisted of vegetation, and of the former only 

 ten per cent. The remaining ninety per cent, of the food of 

 lieterodon was Entomostraca ; but these were not represented at 

 all in the megalops^ the remaining food of these specimens 

 consisting of insects and araphipod Crustacea. 



Sensible and even conspicuous differences in food often 

 appear between groups which are neither widely separate in 

 classification nor yet distinguished by marked differences in 

 alimentary structures, as between species of the same genus. 

 Sometimes these are apparently due to differences in habit with 

 respect to the search for food; but sometimes seem dependent 

 upon distinction of habit or preferences even more obscure. 



Six specimens of the channel cat {Ictalurus pimctatus), 

 taken at Peoria, October 6, 1887, had eaten insects, mollusks, 

 and vegetation at the rate of forty-one, nineteen, and forty 

 per cent, respectively, the vegetation being nearly all Cladoph- 



* See the preceding page. 



