672 THE POPULAR SCIENCE MONTHLY. 



INTRODUCTION AND SUCCESSION OF VERTEBRATE 



LIFE IN AMERICA. 1 



By Professok O. C. MARSH. 

 [concluded from march number.] 



IT remains now to consider the highest group of the animal king- 

 dom, the class Mammalia, which includes man. Of the existence 

 of this class before the Trias we have no evidence, either in this country 

 or in the Old World, and it is a significant fact that, at essentially the 

 same horizon in each hemisphere, similar low forms of mammals make 

 their appearance. Although only a few incomplete specimens have 

 been discovered, they are characteristic and well preserved, and all 

 are apparently Marsupials, the lowest mammalian group which we 

 know in this country, living or fossil. The American Triassic mam- 

 mals are known at present only from two small lower jaws, on which 

 is based the genus Dromatherhcm, supposed to be related to the 

 insect-eating Myrmecobius, now living in Australia. 



Although the Jura of Europe has yielded other similar mammals, 

 we have as yet none of this class from that formation ; while, from 

 rocks of Cretaceous age, no mammals are known in any part of the 

 world. This is especially to be regretted, as it is evidently to the Cre- 

 taceous that we must look for the first representatives of many of our 

 present groups of mammals, as well as for indications of their more 

 ancient lineage. That some discovery of this nature from the Creta- 

 ceous is near at hand, I cannot doubt, when I consider what the last 

 few years have brought to light in the Eocene. 



In the lowest Tertiary beds of this country a rich mammalian 

 fauna suddenly makes its appearance, and, from that time through the 

 age of mammals to the present, America has been constantly occupied 

 by this type of life in the greatest diversity of form. Fortunately, a 

 nearly continuous record of this life, as preserved, is now accessible 

 to us, and insures great additions to our knowledge of the genealogy 

 of mammals, and perhaps the solution of more profound problems. 

 Before proceeding to discuss in detail American fossil Mammalia, it 

 is important to define the divisions of time indicated in our Tertiary 

 and Post-Tertiary deposits, as these in many cases mark successive 

 stages in the development of the mammals. 



The boundary-line between the Cretaceous and Tertiary in the re- 

 gion of the Rocky Mountains has been much in dispute during the 

 last few years, mainly in consequence of the uncertain geological bear- 

 ings of the fossil plants found near this horizon. The accompanying 



1 An address delivered before the American Association for the Advancement of Sci- 

 ence, at Nashville, Tenn., August 30, 1811, by Prof. 0. C. Marsh, Vice-President. 



