154 THE ARCTOGyEIC REALM. [CHAP. 



all the Primates belong to the lemuroid section, and include no 

 monkeys or apes ; and the carnivores are represented solely by 

 the extinct creodont group, which differs from the existing members 

 of the order by the simpler and more primitive structure of the 

 limbs and teeth. The ungulates, again, belong exclusively to two 

 extinct suborders, respectively termed the Condylarthra and the 

 Amblypoda, both of which are very primitive types, with five-toed 

 limbs of simple structure, the former still retaining evidences of 

 affinity with the early carnivores. The tillodonts are quite unlike 

 any existing forms, having a pair of incisor teeth similar to those 

 of rodents in the front of the jaws, while their cheek-teeth recall 

 those of the ungulates. 



All the Puerco mammals are characterised by the lowness of 

 the crowns of their molar teeth, which carry simple tubercles, 

 generally arranged in a triangle ; this type of tooth being known 

 as the tritubercular, and occurring in all the orders found in the 

 Puerco. It will be unnecessary to mention the names of the 

 genera occurring in this horizon, and it will suffice to state that 

 while peculiar to these beds, many of them belong to families ' 

 characteristic of the overlying Tertiaries. Thus we have the 

 Anaptomorphida among the lemuroids, the Arctocyonida, Mesony- 

 chidce, Proviverridce, and Miacida in the carnivores, and the 

 PhenacodontidcR among the condylarthrous ungulates. The Cernay- 

 sian fauna is mostly represented by such fragmentary specimens 

 that the determination of the affinities of its members is a matter 

 of considerable difficulty; but the forms were all more or less 

 nearly allied to those of the Puerco, and the creodont genus 

 Dissacus is common to the two formations ; while Arctocyon, 

 which is met with in the Cernaysian, also occurs in higher horizons 

 both in Europe and America. 



A very remarkable fact connected with the Puerco fauna is 

 that out of the 39 generic types by which it is represented, only 

 eight are followed by analogous forms in the overlying Wahsatch 

 beds, three of which became extinct in the still higher Bridger 

 deposits. This leads Messrs Osborn and Earle to the conclusion 

 that this early mammalian fauna was a kind of failure as regards 

 development, and that only a few of its less specialised members 

 persisted to give rise to the mammals of later periods. 



