5 66 



UNGULATES. 



into the median valley. The second upper and the third lower incisors formed a 

 pair of permanently growing tusks, which were, however, not fully developed till 

 late in life. 



Even more strange than the toxodon was the smaller typothere of the same 

 region, which represents a remarkable approximation in the characters of its skull 



and teeth to the Rodents. While the 

 %^M molars were not unlike those of the 



toxodon, the upper incisors were 

 reduced to a single chisel-shaped pair, 

 and there were no tusks in either 

 jaw. The lower jaw carried one large 

 pair of chisel -like incisor teeth, 

 behind which there came a much 

 smaller second pair. The typothere 

 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 

 characters 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 

 completely 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. 



UNDER-SURFACE OF SKULL OF THE NESODON {\ nat. Size). 



