CIV FISHES OF ILLINOIS 



rences in the northern part of the state. That highly interesting 

 and important peculiarities of distribution are concealed by this 

 gross method of comparison is made evident by an examination 

 of the maps of the distribution of our collections of the various 

 species accompanying this report, where the data are presented 

 in a way to show, not the number of collections, it is true, in 

 which each species was represented, but the number and dis- 

 tribution of localities from which the species has been obtained. 

 From such a study of these maps it appears that the northern half 

 or two thirds of this state is more favorable to a considerable 

 number of species than the southern part, since these species have 

 been taken there in a much larger number of localities ; and also 

 that a small group of species of wide general distribution has been 

 found by us with surprising frequency in the Wabash drainage 

 in this state as compared with that of adjacent districts. 



The preference of certain species for the northern part of 

 Illinois over the southern is clearly illustrated by the distribution 

 maps of the following fifteen species : Noturus flavus Carpiodes 

 thompsoni, Notropis cayuga, N. hudsonius, N. rubrifrons, Hybop- 

 sis dissimilis, H. kentuckiensis , Fundulus diaphanus, Percopsis 

 guttatus, Eupomotis gibbosus, Stizostedion canadense, Perca 

 flavescens, Etheostoma zonale, Roccus chrysops, and Morone in- 

 terrupta. With few and slight exceptions, all the species of this 

 varied list, representing eight families and twelve genera, are 

 so definitely lim'ted to the northern half of this state that one 

 gets the impression, as he examines these maps in 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 

 limited 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 distribu- 

 tion maps a map of the surface geology of the state (Map III.) 

 that we find a plausible explanation of a part, at least, of this 

 peculiar distribution, for all but one of the species above men- 

 tioned are wholly excluded from the area of this glaciation, and 

 this excepted species (Hybopsis dissimilis) appears in but one 

 locality within the lower glaciation, and that a short distance 

 within its border, on the upper Kaskaskia. 



