the Cretaceous-T ertiary Problem. 215 



considerable minority of archaic orders which disappear 

 in the course of the Eocene. Marsupials are rare, multi- 

 tuberculates have disappeared. The reptiles are repre- 

 sented by modern families of crocodiles (Crocodilidse), 

 chelonians (Emydidse, Testudinidse) etc., the Mesozoic 

 groups having mostly disappeared, except that as with 

 the mammalian orders some of the Cretaceous turtles 

 survive, the Bsenidse to the end of the Eocene, the 

 Dermatemydid^ (rare) and Trionychidae (common) to the 

 present day. 



General Relations of the Faunas in the Disputed Formations. 



The formations that intervene between the undisputed 

 Cretaceous and undisputed Tertiary are mostly non-ma- 

 rine. They fall into two groups, one characterized as to 

 its vertebrates by dinosaurs and metatherian mammals, 

 the other by archaic placental mammals and no dinosaurs. 

 The former have been generally referred to the Cre- 

 taceous by vertebratists, the latter to the Tertiary, but 

 distinguished as Paleocene in recent years as the marked 

 faunal difference from the true Lower Eocene came to be 

 better understood. It has been assumed by most writers 

 that the dinosaur faunas were all older than the placen- 

 tal mammal faunas, and the line between the Cretaceous 

 and Tertiary has been drawn between them. In my 

 paper of 1914 I analyzed the evidence and showed that 

 this was not really proven, and that there was reason to 

 suspect that the Paleocene faunas were partly con- 

 temporary with the latest dinosaur faunas, representing 

 a different faunal facies rather than a real change of 

 fauna. I also pointed out the marked distinction between 

 the archaic placental mammals of the Paleocene and the 

 modernized placental mammals of the true Eocene. I 

 have no reason to change any of the conclusions there set 

 forth, but the evidence in support of them has been 

 extended and confirmed in certain particulars. 



c The relationship of the Eocene rodents to the multitudinous later groups 

 is in dispute. 



d The rhinoceroses probably branched off from the Lophiodontidse. They 

 first appear in the upper Eocene. 



e The higher Artiodactyla appear to have evolved during the Tertiary from 

 primitive Old World stocks of Eocene and later age. They are not descended 

 from those two families. 



