428 THE SUCCESSION OF THE VERTEBRATE FAUNAS. 



edentates occur {Megatherium, Mylodon, Glyptodon), these 

 having apparently wandered northwards on the emergence of 

 the isthmus of Panama in the early part of the Pliocene period. 



North American v. Old World Mammals. 



Having thus traced the series of mammalian faunas through 

 the Tertiary formations both of the Old World and of North 

 America, it is interesting to speculate as to where the various 

 elements arose. At the base of the Eocene it is evident that 

 the faunas of the east and the west were essentially identical. 

 As they are traced upwards they gradually diverge. 



The first noteworthy difference is the great development of 

 the Condylarthra in America, and the rise in the Eocene of the 

 large specialized Amblypoda, of which only a single genus 

 (Coryphodon) has been found in the corresponding fauna of 

 Europe. On the other hand, the still larger ungulates of the 

 sub-order Proboscidea seem to have originated in the Old 

 World and did not reach America until the late Pliocene. 



The Perissodactyla the tapirs, rhinoceroses, and horses 

 appear to have advanced on a parallel course on the two conti- 

 nents ; though in America both the rhinoceroses and horses 

 became extinct at the close of the Pliocene, the former without 

 acquiring the characteristic horn. 



Among Artiodactyla, both the Cervidse and Suidse seem to 

 have been approximately parallel in their development on both 

 continents, only differing in some minor branches which soon 

 became extinct. The Camelidse, however, are clearly American 

 throughout, only wandering into the Old World by Asia in the 

 Pliocene. It is almost equally probable that the Bovidae origi- 

 nated in the Old World. 



Among Carnivora, the Creodonta are both American and 

 European ; but on the former continent they only pass upwards 

 into the Canidae, Mustelidse, and the aberrant cats of the- 

 family Nimravidas, while in Europe they are succeeded not 

 merely by these families, but also by the Viverridse, HyasnidaB, 

 Felida?, and Ursidse. The viverroids and hyaenas never reached 

 America, but the true cats and bears arrived in that continent 

 at the close of the Pliocene. 



