CENTRARCHIDvE THE SUNFJSHES 235 



the two crappies (especially sparoides), and the bluegill ; those of the 

 smal er rivers and creeks are the rock bass, the long-eared sunfish 

 and Lepomis humilis; and a special creek species is the green sun- 

 fish (L. cyanellus), the usual sunfish of the smaller prairie streams of 

 central Illinois. 



These differences of local situation and affiliation are most evi- 

 dent in our miscellaneous collections distributed over the minor 

 waters of the state, and such distinctions disappear largely in the 

 Illinois River, which seems to serve as a kind of reservoir or metrop- 

 olis for the fish population of the country, in which its various ele- 

 ments 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 num- 

 ber — 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 per cent, of these 76 Illi- 

 nois River collections contained also the bluegill, while only 27 per 

 pent, of the 94 collections outside these points contained both 

 species. That is, local differences of distribution, signifying ecolog- 

 ical 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 CENTRARCHIDjE found in Illinois 



a. Dorsal fin 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 1Q) Pomoxis. 



bb. Dorsal spines 11 to 13. 



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



cc. Anal spines 6, rays 10 or 1 1 Ambloplites. 



aa. Dorsal more than twice length of anal; anal spines 3. 



d. Body comparatively short and deep, depth in adults as a rule more than f 



of length; dorsal fin not deeply emarginate, the shortest spine behind 

 middle of fin more than § height of longest; operculum entire behind, not 

 emarginate, more or less prolonged in a bony process or flap with a. 

 rounded posterior margin. 



