VI INTRODUCTION. 



induced us to give the necessary attention to this department 

 of the survey, would .be uncalled for, and out of place here. 



In the preparation of this volume on the Palaeontology of the 

 survey, it will be observed that almost exclusive attention has 

 been given to the organic remains of the Carboniferous rocks. 

 This has been done, first, because the most important stores of 

 mineral wealth found within the limits of the State, occur in 

 these rocks; second, because they occupy far the larger portion 

 of its area ; and third, because these strata contain more organic 

 remains, entirely new to science, than airy of our other forma- 

 tions. In a part of a succeeding report, however, now in an 

 advanced state of preparation, it is intended, should the Legis- 

 lature make an appropriation for its publication, to give figures 

 and descriptions of enough of the characteristic fossils of the 

 other formations to give completeness to the whole, and afford 

 students and intelligent miners the means of determining to 

 what geological period the rocks at any outcrop belong, in any 

 part of the State to which their attention niay be directed. It 

 will not, however, be necessary to occupy so much space, in any 

 future report, with figures and descriptions of fossils, as was re- 

 quired here in laying, as it were, a foundation for the whole work. 



It will be observed that, in a few instances, we have figured 

 and described fossils from adjoining States. This, however, 

 has only been done where the same formations occur in Illinois, 

 but have not yet afforded us as perfect specimens of the particu- 

 lar species as those obtained at localities outside of the State. 



The fact that this volume consists of several distinct memoirs, 

 prepared by parties working to a considerable extent indepen- 

 dently, has prevented that degree of uniformity, in the plan of 

 the whole, that would otherwise have prevailed. As each sec- 

 tion, however, treats upon a different department of Palaeon- 

 tology, and a list of all the species is given at the end of this 

 Introduction, arranged by formations and in accordance with 

 their affinities, as far as they go in each rock, without regard to 



