408 
or local. This state lies on the extreme borders of their proper terri- 
tory, and they are not found more commonly in our waters because 
climatic and other general conditions most favorable to their main- 
tenance, here reach the vanishing point. 
Lists OF SPECIES DISTINGUISHING DIFFERENT DISTRICTS FROM THE ILLINOIS BASIN 
GALENA DISTRICT (2): KASKASKIA RIVER DISTRICT (1): 
Hybognathus nubila (Western) Etheostoma squamiceps (Southern) 
Crystallari lla (South 
rystallarta asprella (Southern) wanheul opis RICE (GN: 
ROCK RIVER DISTRICT (3): Harelipped sucker (rare; Southern) 
Hybognathus nubila (Western) Pigmy sunfish (Southern) 
Hadropterus evides (Western) Eupomotis heros (Southern) 
Crystallaria asprella (Southern) Hadropterus ouachite (Southern) 
Crystallarta asprella (Southern) 
MICHIGAN DISTRICT (9): Etheostoma squamiceps (Southern) 
Brook lamprey (rare) 
Long-nosed sucker (Northern) SALINE RIVER DISTRICT ,(2): 
Whitefish (Great Lakes) Etheostoma obeyense (Southern) 
Lake herring (Great Lakes) E. squamiceps (Southern) 
Lake trout (Great Lakes) 
Great Lake catfish (Northern) CAIRO DISTRICT (8): 
Nine-spined stickleback (Northern) Brook lamprey 
Cottus ricet (Great Lakes) Hybognathus nubila (Western) 
Uranidea kumlienti (Great Lakes) Long-nosed dace (rare in I1linots) 
Flat-headed chub (Western) 
MISSISSIPPI STRIP (3): Chologaster papilliferus (Gubtseeae 
White sturgeon (rare; Mississippi only) Pigmy sunfish (Southern) 
Hybognathus nubila (Western) Eupomotis heros (Southern) 
Crystallaria asprella (Southern) 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 cen- 
tral district as a type; but a fuller and more accurate idea of the com- 
position 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 uni- 
form 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 occurring within the area of both the districts taken to- 
gether asone. In the Galena district, for example, there are 44 spe- 
cies 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 
