THE PREHISTORIC HORSE 3 



perhaps three million years. Careful investigation and dis- 

 coveries show that animals of the horse class existed in the 

 Lower Eocene period and gradually passed upward through 

 twelve stages of development in the course of geological time, 

 becoming more and more developed toward the present-day 

 type as the geological periods became more and more recent. 

 These various forms properly grouped may be briefly described 

 as follows : 



1. HyracotJierium. Existed in Lower Eocene period. Only 

 the skull has been found. The teeth display six rounded knobs 

 or cusps on upper molars and four on lower. Found only in 

 London Clay, England. 



2. EoJiippus. From Lower Eocene period. Teeth vary some 

 from above. Fore foot has four complete toes and splint of a 

 fifth. Hind foot has three toes and splint. Found in Wyoming 

 and New Mexico. Were about size of fox terrier, and known 

 as "dawn horses." 



3 and 4. Protorohippus and Oroliippus. From Middle Eocene 

 period. In these animals the splints have disappeared, leaving 

 the complete toes, as in Eohippus. The crests on the molars 

 are more apparent, and the last premolar has become almost like 

 the molars. This type was about the size of a small dog, perhaps 

 fourteen inches high. The Protorohippus was discovered in 1880 

 in Wyoming. 



5. Epihippus. From Upper Eocene period. Only incomplete 

 specimens have been found. The teeth have gone through 

 changes with more crescents and crests, with another premolar 

 tooth becoming like the molars. There are four toes in front 

 and three behind, but the central toe in each is larger and 

 stronger and more important than the side toes. 



6 and 7. Mesohippus. From Oligocene period. There are 

 three toes on each foot and a splint representing the fifth toe of 

 the fore foot. The middle toe is now much larger than it was in 

 the preceding, the side toes bearing but little weight of the body. 

 Three of the premolars have also become like full molars. In 

 the Middle Oligocene is found one species about the size of a 

 coyote, or eighteen inches high, while in the Upper Oligocene is 

 another species as large as a sheep. 



