23 



ilar 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 countr}^, living or fossil. The American Triassic Mam- 

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

 which is based the genus JJromotlieriuin^ supposed to be related 

 to the insect-eating Mynnecohins^ now living in Australia. 



Although the Jura of Europe has yielded other similar 

 Mammals, we have as yet none of this class from that forma- 

 tion ; 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 Cretaceous 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 countr}', a ricli ]\[:im- 

 malian fauna suddenly makes its appearance, and from that 

 time through the Age of Mammals to the present, America 

 has been constantly occupied by this tj'pe of life in the greatest 

 diversit}^ of form. Fortunately, a nearly continuous record of 

 this life, as preserved, is now accessible to us, and ensures great 

 additions to our knowledge of the genealogy of Mammals, and 

 perhaps the solution of more profound problems. Before pro- 

 ceeding 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 region of the Eocky Mountains has been much in dispute 

 during the last few years, mainly in consequence of the uncer- 

 tain geological bearings of the fossil plants found near this hori- 

 zon. The accompanying invertebrate fossils have thrown little 

 light on the question, which is essentially, whether the great 



