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smallarea above the mouth of the Illinois, has been repeatedly worked 
over by ice in the course of the successive divisions of the glacial period. 
The oldest glaciated area, known as the lower Illinoisan glaciation, 
covers the greater part of southern Illinois and a narrow belt of the 
southeast part of the central section of the state. Next to this at the 
northwest, and immediately east of the lower half of the Illinois 
River, is the middle [llinoisan; above this, in the west-central part 
of the state, between the Illinois River and the Rock, is the upper 
Illinoisan; and still farther north, in the Rock River basin, are the 
Iowan and Preiowan glaciations, reaching northward across the Wis- 
consin boundary. East of the last three mentioned, and north of the 
southern Illinois district, the Wisconsin glaciation, the most recent 
of the series, covers about a fourth of the state. It is to the peculiar 
features of the lower Illinoisan glaciation especially that we shall 
presently be compelled to pay particular attention, because of their 
evident effect on the distribution of a considerable group of our 
fishes. 
The topographical relations of the state to the surrounding terri- 
tory are as simple and open as its own interior hydrography, and 
there is little to suggest the possibility of anything in the least pecul- 
iar in the general constitution or the relations of its fauna, or any- 
thing problematical or especially interesting in the details of the dis- 
tribution of its native fishes. We shall find reason to believe, how- 
ever, that this appearance is misleading, and that the subject, stud- 
ied in detail, contains matter of unusual interest, and presents prob- 
lems of considerable difficulty, a solution of which will lead us to 
some novel results. 
It is true, however, generally speaking, that the distribution of 
Illinois fishes reflects, in uniformity and relative monotony, the fea- 
tures of the topography of the state. A few species occurring in Lake 
Michiganand characteristic of the Great Lakes are, in fact, the only Ill- 
nois fishes which are definitely and permanently separated from their 
fellows in other Illinois waters by what may be called geographical 
conditions, and these conditions are not physical obstacles to their 
passage from Lake Michigan to the Illinois River. 
Excluding, for the moment, these fishes special to the Great 
Lakes, we find elsewhere in Illinois a general commingling and over- 
lapping of the fish population of the surrounding territory, the limits 
