GENERAL AND INTERIOR DISTRIBUTION XCIX 



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 collec- 

 tions, it is true, in which each species was represented, but the 

 number and distribution 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 drain- 

 age 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 thompsont, 

 Notropis cayuga, N. hudsonius, N. rubrifrons, Hyhopsis dissimilis, 

 H. kentuckiensis , Fundtdus diaphanus, Percopsis guttatus, Eupomotis 

 gibbosus, Stizostedion canadense, Perca flavescens, Etheostoma zonale, 

 Roccus chrysops, and Morone interrupta. 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 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 lim- 

 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 w4th 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 



