XCviii FISHES OF ILLINOIS 



CAIRO DISTRICT (8): 



Brook lamprey Chologaster papttliferus (subterranean) 



Hybognathus nubtia (Western) Pigmy sunfish (Southern) 



Long-nosed dace (rare in Illinois) Eupomotis heros (Southern) 



Flat-headed chub (Western) Etheostoma squamiceps (Southern) 



RELATIONS OF EACH DISTRICT TO ALL THE OTHERS 



In the foregoing discussions and analyses the fishes of the 

 various districts have been compared with those of the largest 

 and most central district as a type; but a fuller and more accurate 

 idea of the composition of the fish population of Illinois and of 

 its relations in the various hydrographic divisions of the state 

 may be obtained by a comparison of the species of each of our 

 ten districts successively with those of all the others. This 

 may be done in an exact and uniform manner by determining 

 for each pair of districts the ratio which the number of species 

 common to the pair bears to the whole number of species occur- 

 ring within the area of both the districts taken together as one. 

 In the Galena district, for example, there are 44 species recorded, 

 and in the Saline River basin there are 55, a total of 99; but as 

 26 of these species have been found in both these districts, this 

 number has been taken twice in the above addition, and the 

 number of species found by us in the entire area of these two 

 districts is consequently 73. The ichthyological affinity of these 

 two areas is evidently to be measured by the ratio which the 

 number of species common tq both bears to the whole number 

 of species found in either or both the areas in this case, the 

 ratio of 26 to 73, or 36 per cent. That is, 36 per cent, of the 

 fishes found in either of these two districts have been found by 

 us in both of them. 



A similar analysis of the data for each of the forty-five pairs 

 which it is possible to make up from our ten hydrographic dis- 

 tricts, yields the material for the following table of common 

 species and of ratios of affiliation. This table shows, in the 

 lower left-hand part, the number of species common to each 

 pair of districts, and in the upper right-hand part the ratios 

 which these numbers bear to the number of species occurring 

 in each pair of districts taken as one. The number of species 

 common to any two districts will be found in the lower left- 

 hand part of the table, where the column for one district inter- 

 sects with the line for the other, and the ratio of affiliation for 

 the same pair of districts will be found in the opposite part of 

 the table at the intersection of the line for the first with the 

 column for the second. A simple inspection of the figures in the 



