EARLY TERTIARY MAMMALS IOI 



be devoid of mammalian remains. This gap, however, has been 

 filled up by the discovery of mammalian remains in the North 

 American Laramic formation, which seems to be clearly of Creta- 

 ceous age. Furthermore, it is held by some that the Purbeck beds 

 are more properly to be placed with the Cretaceous, which would 

 then necessitate the consideration under the present heading of 

 some of the types already dealt with ; and if, as is suggested in 

 the following section, the lowest so-called Eocene beds are really 

 referable to the Cretaceous, there is no lack of mammalian remains 

 in that period. And, moreover, it was in that case the Creta- 

 ceous period which witnessed the evolution of the existing orders 

 of Placental mammals. Otherwise the mammalian remains of the 

 Cretaceous agree with those of the Jurassic. We find remains 

 of the Multituberculata in fragments of Plagiaulacidae and 

 Polymastodontidae. Ptilodus is a genus which has two pre- 

 molars; and Meniscoessus is another multituberculate from the 

 same Laramic formation. The other detached fragments of 

 mammals are thought by Osborn to represent both Placentals 

 and Marsupials. 



The Mammals of the Tertiary Period. Unless the lowest 

 beds of the earliest Tertiary period, the Eocene, such as the 

 Torrejon of North America, should be in reality referred to the 

 Cretaceous, there is no evidence that the modern groups of 

 Mammalia existed before the present epoch of the earth's history. 

 It is probable, however, that the Eutheria as a group were Meso- 

 zoic. The fossil jaws that have been considered in the last chapter 

 may quite probably be primitive Eutherians, or even divisible, as 

 believed by Professor Osborn, into Marsupials and Insectivores. 

 In the Tertiary, however, apart from the question as to the 

 nature of the Puerco and Torrejon formations, and as to certain 

 South American strata whose fossil contents have been investi- 

 gated by Professor Ameghino, we find the first traces of mammals 

 definitely referable to existing orders, or to be distinctly com- 

 pared with existing orders. Since, however, representatives of 

 types which have obvious relationships to modern types appear 

 in considerable profusion in the very earliest strata of the 

 Eocene, it seems clear that much remains to be discovered in 

 beds earlier than these. Confining ourselves, however, to facts 

 and to comparisons which can be made on more than a few 

 lower jaws and scattered teeth, which is practically all that we 



