PEEFACE. - XXxiii 



to be the type of the restricted genus bearing the old name, it is not com- 

 petent for a third author to discard this and select another of the original 

 species as a type, when by so doing changes are necessitated in nomenclature. 



6. When a generic name has lapsed from sufficient cause into synonymy, 

 it need not be thenceforth entirely rejected from nomenclature, and may 

 still be applicable to any new and valid genus. 



The earlier pages of this volume were printed between two and three 

 years prior to the greater part of it, hence some of the earlier statements 

 will be found to be modified in the more detailed discussions which follow. 

 One such point is the distinction which should be maintained between the 

 John Day and White River epochs; another point is the great distinction 

 which should be recognized to exist between the Puerco and later Eocene 

 periods. The faunae of the Puerco and Wasatch epochs are as diverse from 

 each other as are those of the Bridger and White River. 



Some inequalities in the text, and the intercalation of numerous plates 

 which carry letters attached to their numbers must be explained. These 

 peculiarities are due to the fact that the discovery of the Puerco fauna was 

 made after the first pages of the volume had been struck off, and the greater 

 number of the plates had been numbered and printed. 



The present volume includes the vertebrata of the Eocene and of the 

 Lower Miocene, less the Ungulata. There are described three hundred and 

 forty-nine species, of which I have been the discoverer of all except thirty- 

 two. They are referred to one hundred and twent3^-five genera. 



The most important results which have accrued to paleontology through 

 the researches here set forth, are the following: 



(1.) The discovery of the Laramie genus Champsosaurus in Ter- 

 tiary beds. 



(2.) The discovery of Plagiaulacidce, in Tertiary beds. 



(3.) The discovery of the characters of five families and many genera 

 and species of the Creodonta. 



(4.) The discovery of the characters of the Periptychidoe and its included 

 genera; and 



(5.) Of the Meniscotheriidce ; and 



(6.) Of the Phenacodontidce and its genera. 



Ill (; 



