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 
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 
under-subface of skull of the nesodon nat. size), completely cut oft 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. 
