1154 



differed from all living Ungulates, and thereby again resembled Rodents, in having 

 collar bones (clavicles). 



Finally, certain animals from the Eocene of North America, known as tillodonts, 

 seem to combine the characteristics of the modern Ungulates, Carnivores, and 

 Rodents, and thus almost defy classification. 



The occurrence of all these remarkable Ungulates, so utterly different from 

 those of all other parts of the world, indicates that during the Miocene period 

 South America, with its many peculiar types of Edentates, must have been com- 

 pletely cut off from the northern half of the continent. During the later Pleistocene 

 period, the two areas must, however, have become connected, since at that epoch 

 we first meet with horses, deer, llamas, and other northern types in South America; 

 while some southern forms obtained an entrance into North America. 



