TJw Food of Fishes. 45 



the Crustacea rise to seventy-nine. Here, however, dif- 

 ference of locality interferes to prevent any satisfactory 

 comparison with other ages, — as these specimens were all 

 taken in August, from Calumet River, at South Chicago. 

 This slow stream, clogged with Algt^ and a great variety 

 of other aquatic plants in midsummer, also swarms with 

 Crustacea, — especially the little Allorchestes dentata. This 

 species made sixty-three per cent, of the food of these 

 specimens ; and an undetermined species of Asellus, four- 

 teen per cent. A few Ganimarus fasciatus were also found. 

 The insects were Corixa and larv.v" of AgrionidtV. 



It will be seen that, excepting the gradual increase of 

 the number of fishes eaten, these data show no especial 

 difference in the young of different ages. Smaller speci- 

 mens and a larger number from a greater variety of situ- 

 ations, would be necessary to exhibit this difference. 



The food of the young as a whole, apparently, does not 

 differ essentially from that of the large-mouthed species, 

 except in the probably greater importance of the insect ele- 

 ment — especially Corixas, which in these twenty-four 

 specimens amounted to fifty per cent, of the food — and the 

 inferior importance of fishes. 



This peculiarity is expressed in a slightly different man- 

 ner in the food of the adult. The three specimens exam- 

 ined had eaten only fishes {Noturus ffavus and Percina 

 caprodes) and crawfishes {Canihanis ppoplnquas).! — thirty- 

 eight per cent, of the former and sixty-two per cent, of 

 the latter. 



This is the first of several instances in which the ratio 

 of fishes in the food of allied species and genera was found 

 to correspond to the size of the mouth, being largest in 

 those with the largest oral opening.* 



*The frequency with which these two species of black bass are con- 

 founded makes it desirable that a single reliable character should be 

 selected by which they can be invariably distinguished, whatever the 

 age of the specimen. This character is afforded by the size of the scales, 

 the small-mouthed species having the smaller scales. In this species 

 there are eleven longitudinal rows of scales between the dorsal fin and 

 the row of perforated scales running along the middle of the side, called 

 the lateral line. In the large-mouthed species, there are never more 

 than nine such rows. The young are easily distinguished by the longi- 

 tudinal black stripe along the side of the large-mouthed bass, which i§ 

 wanting in the young of the other species, 



