THE MIOCENE PERIOD. 



287 



and other genera flourished and deployed into forty or more species. 

 They were still three-toed, but the two lateral toes were much reduced 

 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 



Fig. 455. — An American Miocene Camel, Oxydactylus longipes Peterson, from ]the 

 Loup Fork beds of Nebraska. (After Peterson.) 



evolving horse to the open dry plains and their grassy food (Fig. 456). 

 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 of the leg bones, were modifica- 

 tions in the interest of combined speed and strength. A corresponding 

 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 supporting dentine and cement 

 on either side. The teeth were also gradually elongated to provide 



