420 
of muddy waters, only 10 per cent. of all our data-bearing collections 
of this group coming from such situations, while 61 per cent. of them 
were from bottoms of rock and sand, and 29 per cent. from those of 
sand and mud. _ It is consequently clear that the suspended detritus 
of the streams of southern Illinois and the clay and mud of which 
their banks and bottoms are commonly composed, are an important 
part, at least, of the cause of the smaller variety of fishes in these 
waters; and these conditions trace back through the character of the 
soil to the geological history of the central part of southern Illinois. 
FISHES OF THE OHIO AND OF THE MISSISSIPPI DRAINAGE 
A comparison and classification of our distribution maps from 
another point of view enables us further to distinguish two rather 
definite groups of species coincident in great measure, but not wholly 
so, with the two groups which we have found in an opposite relation 
to the lower Illinoisan glaciation. No less than 27 of our species 
have either an exclusive or at least a strongly preponderant dis- 
tribution in the Mississippi drainage in the western and northern 
parts of the state, while 8 species, on the other hand, are very defi- 
nitely preponderant in the Ohio drainage in the southern and eastern 
parts. Nineteen of the 27 species of the first list are also on the list 
of species excluded from the region of the lower I]linoisan glaciation, 
while 6 of the 8 species of the second list are also on that of species dis- 
tributed freely through this southern Illinois district. We have evi- 
dence here of another influence strongly affecting distribution, coin- 
cident in part with that already discussed, but independent of it also 
in part, the two causes, or sets of causes, operating together to deter- 
mine the actual range of most of the species of limited distribution in 
this state. 
The impression produced by an examination of the two sets of 
maps for the fishes above mentioned, is that of a small group of spe- 
cies, on the one hand, which enter the state from the south and east 
by way of the Wabash and the smaller tributaries of the Ohio, and, 
on the other hand, of a much larger group, most of which have en- 
tered the state from the west and north, making their way to its in- 
terior mainly by the Illinois and the Rock, but sometimes by the 
Kaskaskia and the Big Muddy also. Species of the Ohio group 
sometimes seem to spread into the headwaters of adjacent streams, 
