466 Illinois State Laboratory of Natural History. 



pal features of the food. The relatively minute size of the ob- 

 jects on which it feeds, the absence of mud from its intestine, 

 and its seemingly positive preference for animal food, indicate 

 that it is not only able to gather large quantities of very mi- 

 nute objects among the weeds and from the muddy bottom 

 without filling itself with mud, but that it can separate the 

 Entomostraca from the Algas among which they swim. I 

 cannot see how this is done unless its paddle be used to stir up 

 the weeds in its advance, as it swims along, thus driving up the 

 animal forms within reach of its branchial strainer, while the 

 mud and vegetation settle out of its way. 



What is the meaning of the minute and evanescent teeth 

 on the jaws of Polyodon, I am unable to surmise, but judge 

 that they can only be accounted for by reference to primitive 

 conditions of life of which the present habits of the fish give 

 us no hint. 



Eight specimens obtained from Peoria, Pekin, and Henry 

 on the Illinois, from the Ohio River at Cairo, and from the 

 Mississippi at Quincy, in six different years, will probably suffice 

 to give a fair general idea of the food, taken in connection with 

 suggestions made above, based on a study of the structures of 

 alimentation. 



The vegetable elements of the food were eaten by four of 

 the specimens, and amounted to only seven per cent. It is to be 

 noted, however, that one of the specimens taken at Quincy 

 had derived thirty per cent, of its food from a species of Nostoc, 

 while another, taken at Peoria in May, had found about one 

 fifth of its food among vegetable objects. A little Potamo- 

 geton, some filamentous algae and diatoms, together with a small 

 amount of terrestrial rubbish, were the elements recorded. 



Fishes and mollusks were without representation in the 

 alimentary contents of these specimens; while insects and crus- 

 taceans made by far the larger part of the food, — the former 

 taken by all the specimens, and in nearly twice the ratio of the 

 latter. The minor items of this class were Corethra larvae 

 (twelve per cent.) and Chironomus larvae (five per cent.). 

 Larvae of Neuroptera made one half the food, and were eaten 

 by six of the specimens, — Hexagenia larvae alone amounting 

 to forty-seven per cent. A few case-worms (Phryganeidae), 



