416 
gibbosus, Stizostedion canadense, Perca flavescens, Etheostoma zonale, 
Roccus chrysops, and Morone tnterrupta. With few and slight excep- 
tions, all the species of this varied list, representing eight families 
and twelve genera, are so definitely limited to the northern half of 
this state that one gets the impression, as he examines these maps 1n 
succession, that some invisible barrier to their southward dispersal 
exists in the neighborhood of the Sangamon River. 
PECULIARITIES OF DISTRIBUTION IN THE LOWER ILLINOISAN GLACIATION 
That the distribution of these more northerly species is not lm- 
ited by the watersheds is shown by the fact that they range across 
the state indifferently into all the stream systems of northern Illinois. 
It is not until we compare with our distribution maps a map of the 
surface geology of the state (Map III.) that we find a plausible ex- 
planation of a part, at least, of this peculiar distribution, for all but 
one of the species above mentioned are wholly excluded from the 
area of this glaciation, and this excepted species (Hybopsis dissim- 
ilis) appears in but one locality within the lower glaciation, and that 
a short distance within its border, on the upper Kaskaskia. 
Especially significant in this relation are several cases in which 
species of this list range southward in the eastern part of the state 
upon the upper tributaries of the Kaskaskia and the Embarras, for 
in so doing they simply follow southward the course of the Shelby- 
ville moraine which forms the boundary between the Wisconsin and 
the lower Illinoisan glaciations in east-central Illinois. The maps 
for Noturus flavus, Hybopsis dissimilis, H. kentuckiensts, and Strzo- 
stedion canadense are examples. 
That this coincidence of distribution and surface geology points 
to a true explanation is further shown by the maps for twenty-two 
other species which range more definitely to the southward than the 
foregoing twelve, but which nevertheless avoid the southern glacia- 
tion more or less completely and to an unmistakable degree. For 
example, 19 of our 94 collection localities for the hogsucker (Catos- 
tomus nigricans) lie below the Springfield parallel, but only three of 
them are in the lower Illinoisan glaciation, and these are barely 
within its borders. Of our thirty localities for the short-headed red- 
horse (Moxostoma breviceps) only two are in this glaciation, and these 
are near its boundaries on the Embarras and the Kaskaskia. The 
very abundant minnow Campostoma anomalum was taken by us from 
