280 FISHES OF ILLINOIS 



studied, Percina caprodcs and Microperca punchdata, have deserted 

 in great measure the usual situations of the darters, and are fre- 

 quently found among weeds and alga? in comparatively quiet water 

 with a muddy bottom, the others being much more closely confined 

 to swift and rocky shallows. Consistently with this difference, 

 these two widely unlike species agree in their choice of food, wh ch 

 in both consists largely of Entomostraca or other small crustaceans. 

 The larger species had also eaten a few small mollusks (Ancylus). 



Where a group of species has become assimilated by a similar 

 adaptation to a common class of situations, and has thus become a 

 definite ecological assemblage, those in which the adaptive proc- 

 esses have gone farthest are, of course, most likely to be limited to 

 the characteristic situation — are most likely, consequently, to be 

 taken by the collector in each others' company. By applying this 

 rule to an analysis of the collections of darters made in Illinois, 

 we find that the most typical species obtained by us in any consider- 

 able number are the following six, mentioned in the order of the rela- 

 tive frequency of their associate occurrence in our collections : Ha- 

 dropterus phoxocephalus, Etheostoma zonale, Etheostoma flabellare, 

 Hadropterus aspro, Ammocrypta pellucida, and Etheostoma cceritleum. 

 Apparently the least stringently connected with their kind by the 

 associative relationship are Diplesion blennioides, Etheostoma jessicr, 

 Boleosoma camurum, and Boleichthys fusiformis. The species of the 

 second list will presently be seen to be those which have wandered 

 widely from the common field of the subfamily, and which are conse- 

 quently found most frequently in situations to which the other spe- 

 cies rarely resort. Furthermore, in separating themselves from 

 their fellows in respect to local distribution, they have not. as a rule, 

 approached each other, but remain as loosely affiliated ecologically 

 among themselves as they are with the more typical members of the 

 group. A notable exception to this statement is found in Boleosoma 

 camurum and Boleichthys fusiformis, which occur in similar waters, 

 and most abundantly also in the same part of the state. In these 

 two common species the coefficient of association each with the other 

 is unusually high, much higher, indeed, than the average coefficient 

 for the most typical species of the subfamily. 



The darters are distributed through southern Canada and the 

 United States east of the Rocky Mountains and northern Mexico ; as 

 far westward as south Nebraska ; and northward to Qu'App.elle, in 

 the basin of the Red River of the North. There are some eighty or 

 ninety species of this subfamily in North America, and in Illinois 

 twenty-three species belonging to ten genera. The majority of them 



