HISTORY OF THE PERISSODACTYLA 



317 



(a) by making the upper profile strongly concave from before 

 backward ; (6) by greatly widening the top of the cranium, 

 where in the older and more primitive genera the high and 

 thin sagittal crest was placed ; (c) by immensely increas- 

 ing the thickness of the cranial bones and at the same time 



hollowing them by means 

 of an intricate svstem of 



Fig. 165. — Series of heads of t titanotheres in ascending geological order. A., -f Pal- 

 CBOsyops, lower Bridger. B., -\ Mantenceras, upper Bridger. C, f Diplacodon, Uinta. 

 D., t Tikinotherium, extreme development of horns, White River. From models in 

 the American Museum and Princeton University. 



cavities ; in this way sufficient strength was secured without un- 

 due increase in weight. 



(6) The growth of the brain did not keep pace with the 

 increase in the size and weight of the body and head, and this 

 deficiency may have been a factor in determining the early 

 extinction of the family. 



(7) To support the huge head, stout ligaments and power- 

 ful muscles were needed in the neck and trunk and these in 

 turn required large bony surfaces for their attachment. To 

 meet this need, the spines of the anterior trunk-vertebrae were 



