Introduction 



It is the purpose of the present vokime to furnish to those 

 interested in Illinois fishes a reliable guide to a knowledge of the 

 species, a careful account of their local and general distribution 

 and of their relations to their environment, a correct idea of the 

 function and relative importance of the different species in the 

 general system of aquatic life, and a fairly full summary of their 

 habits and utilities so far as these are now known. To this end 

 the species have, with very few exceptions, been described anew 

 from the specimens of our collection, with due use, however, of 

 descriptions already extant; analytical keys have been made, 

 adapted, or selected, with special reference to the Illinois species; 

 and our data of geographical and local distribution and of 

 ecological situation and relationship have been analyzed, to a 

 considerable extent, by statistical methods. 



The collections and field observations of Illinois fishes upon 

 which this report is based were begun by the senior author in 

 1876, and were continued by him and by a considerable list of 

 assistants, at rather irregular intervals, to 1903. With the estab- 

 lishment of the Illinois Biological Station on the Illinois River at 

 Havana in 1894, field work in ichthj^ology became more nearly 

 continuous than had previously been possible. An especially 

 interesting study was made at Havana during the winter and 

 spring of 1898 and 1899 by Mr. Wallace Craig, an assistant of 

 the State Laboratory, to whom was assigned the duty of making 

 systematic collections at fixed points by the uniform use of 

 identical apparatus at each, determining, counting, and recording 

 all the species obtained in each situation. It was the object of 

 this investigation to apply, in the field of ichthyology, the quan- 

 titative method which had been used with distinguished success 

 in the study of the plankton of the Illinois River and adjacent 

 waters at the Havana Station. During the summer of 1899 field 

 work was transferred to Meredosia with Mr. H. A. Surface in 

 charge, and later it was taken up by Mr. Thomas Large at Mere- 

 dosia and Ottawa, to which latter place the station equipment 

 was transferred in 1901. Extensive wagon-trips were made from 

 time to time through various parts of the state for a study of the 

 fishes of the smaller streams, the most important of them in 1899 

 by Mr. Large, to whom we are indebted for the field determi- 

 nation of many of our specimens and for numerous descriptive 

 notes on the waters and situations visited. 



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