CEiSTTRAECHID^. — ^THE SUNFISHES 335 



guished by their greater frequency in lakes and ponds; while 

 the warmouth and Lepomis humilis are especially noticeable 

 because of their high frequencies over a muddy bottom. The 

 principal species of the larger rivers are the two crappies (es- 

 pecially sparoides) and the bluegill; those of the smaller rivers 

 and creeks are the rock bass, the long-eared sunfish and Lepomis 

 hu77iilis; and a special creek species is the green sunfish (L. 

 cyanellus), the usual sunfish of the smaller prairie streams of 

 central Illinois. 



These differences of local situation and affiliation are most 

 evident in our miscellaneous collections distributed over the 

 minor waters of the state, and such distinctions diappear largely 

 in the Illinois River, which seems to serve as a kind of reservoir 

 or metropolis for the fish population of the country, in which its 

 various elements unite and mingle in a relatively indiscriminate 

 way. This fact appears especially on a comparison of the data 

 of the collections made at Meredosia and at Havana — about a 

 third of our whole number — with those made outside. Thus, 

 76 of our 170 collections of the pale crappie were made at either 

 Havana or Meredosia, and 94 of them came from other places. 

 Fifty-five percent, of these 76 Illinois River collections contained 

 also the bluegill, while only 27 per cent, of the 94 collections 

 outside these points contained both species. That is, local 

 differences of distribution, signifying ecological distinctions, 

 were twice as evident in the collections made from the smaller 

 waters as from those made from the Illinois. 



In addition to these distinguishable differences of local prefer- 

 ence, the sunfishes are more strongly differentiated than usual 

 with respect to their feeding structures — the mouth, the gill- 

 rakers and the pharyngeal teeth. Those with large mouths 

 have a large ratio of fishes and crawfishes in the food, those with 

 long gill-rakers take more Entomostraca, and those with broad 

 and heavy pharyngeal bones, bearing stout blunt teeth, live more 

 largely on mollusks. Additional details on this topic will be 

 found in the discussion of the several genera and species. 



Key to Genera of CENTRARCHID^ found iisr Illinois 



a. Dorsal fln little longer than anal, if any, its length 1 to 1.4 times length of 



anal base; anal spines 5 to 8 in number. 



b. Dorsal spines 5 to 8 (occasionally 9 or even 10) .' . . Pomoxis. 



bb. Dorsal spines 11 to 13. 



c. Anal spines 7 or 8 (occasionally 6), the rays 13 to 15 Centrarchus. 



oc. Anal spines 6, rays 10 or 11 Ambloplites. 



