THE MIOCENE PERIOD 821 



the two lateral toes were dwarfed and did not usually touch the 

 ground, while the central one was strengthened and bore all the 

 weight. A large group of structural features were being modified 

 concurrently with the feet, to fit the evolving horse to dry plains 

 and grassy food (Fig. 551). The elimination of the side toes, the 

 lengthening of the limbs, the change of the joints to the "pulley- 

 wheel" type, the concentration of the limb muscles near the body 

 to reduce the weight of the parts most moved, and the consolidation 



Fig. 550. An American Miocene Camel, Oxydactylus longipes Peterson, 

 from the Loup Fork beds of Nebraska. (Peterson.) 



of the leg bones, were modifications in the interest of speed and 

 strength. An elongation of head and neck was necessary to reach 

 the ground. The front teeth were reduced to chisel-like, cropping 

 forms, somewhat resembling those of the rodents, while the molars 

 evolved a tortuous distribution of the enamel so flanked by dentine 

 and cement that the differences of wear gave rise to ridges of 

 enamel suited to grinding, and protected against breaking by sup- 

 porting dentine and cement on either side. The teeth were also 

 (gradually elongated to provide for the great wear caused by the 



