INTRODUCTORY. • VII 



their distribution in the several memoirs, little inconvenience 

 can arise from the want of a more systematic arrangement of 

 the contents of the volume. 



In the portion of the work devoted to the description of the 

 Vertebrates, the species are described much in the order that 

 the figures were most conveniently arranged and numbered on 

 the plates, and not always in accordance with their zoological 

 or geological relations'; though the locality and position of each 

 species is mentioned at the end of its description, and a syste- 

 matic synopsis of the species is given at the close of the memoir. 



The Invertebrates, excepting the Polyzoa (which are all de- 

 scribed together in a supplementary paper) , are described in the 

 order of their geological succession, and arranged zoologically, 

 as far as they go in each formation. The fossil plants are also, 

 like the Polyzoa, described together in a separate section, mainly 

 in the order of their botanical relations. The geological position 

 and locality of each species, however, is mentioned in connection 

 with its description. 



The whole number of species of all kinds, illustrated and 

 described, is about 325, of which nearly 300 were discovered or 

 first made known to science through the agency of the Illinois 

 Geological Survey. Of these 325 species, 50 are plants, 15Q 

 Invertebrate animals, and 119 Vertebrates. Altogether they 

 represent 115 genera, 25 of which have been established by 

 parties connected with the survey. Of these genera 18 are 

 plants, 67 Invertebrates, and 30 Vertebrates— the latter of 

 which, with one exception (a BatracMan) , being all fishes. Of 

 the 115 genera represented, 81 are more or less fully characteiv 

 ized, and most of them illustrated in this volume.* 



The various rocks from which these fossils were obtained 

 having been described, and their relations fully discussed, in the 



* As no other single volume of Palaeontology, hitherto issued in this country, eon- 

 tains so large a number of generic descriptions, and as most of these descriptions, even 

 of previously established genera, have been carefully rewritten, and more or less modi- 

 fied from the study of extensive collections of specimens, it i3 hoped that this feature 

 of the Report will add materially to its value and usefulness, especially to studems who 

 may not have access to extensive libraries of works on Palaeontology. 



