184 



DINOCERATA. 



During- tliese two stages of modification, or at a later period, a 

 reduction in tlie luimber of digits in some forms also took place, evidentlv 

 as a result of the same causes. As progression on dry land with the 

 plantigrade five-toed foot began, the first digit, being the shortest of the 

 series, soon left the ground, and was gradually lost. 



yiODRE 150. — Left fore foot of Coryphodon hamatn.f, Marsh, front view. 

 Figuhe ]51 — Left liind foot of same. 



Bolli liy-iires are one-tliird natural size. 



Fk;. 152. 



m 



Figure 152.— Lc-(t lore foot of Bin 

 Figure 153. — Left hind foot of same, 



Both fi,i,'iircs .ore one-fifth natural size. 



The four remaining digits, luning to do the work of five, were 

 strengthened by the interlocking alread}- mentioned, and also bv coming 

 nearer together. 



(4.) In the next change that occurred, two kinds of reduction 

 began. One leading to the existing Perissodactyl foot, and the other, 

 apparently latoi', resulting in the Ai-tiodactyl type. In the former, the 

 axis of the foot remained in the middle of the third digit, as in the 



