The Food of Fishes. 
45 
cent., but the insects drop away to seven per cent., and 
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 Algse 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 Gammarus fasciatus were also 
found. The insects were Corixa and larvse of Agrionidse. 
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 
element — 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 
manner in the food of the adult. The three specimens ex- 
amined had eaten only fishes ( Noturus flavus and 
Percina caprodes) and crawfishes ( Gambarus propin- 
quus) — 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 larg- 
est 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 longitudinal 
black stripe along the side of the large-mouthed bass, which is wanting 
in the young of the other species. 
