Kvoi.rTioN OF Tin-: horse. 119 



the most jirimitix-e sla.^e known in the horse's ancestral line. 

 The molar teeth have six cusps on the upper and four on the 

 lower ones, and these are just beginning to fuse into crests. 

 In each jaw the fourth premolar has three cusps, the third 

 two, and each of the first and second premolars one. The 

 teeth are short crowned and like those of omnivora. The fact 

 that this skull is more primitive than any yet found in America 

 would seem to indicate that the original stock was Eurasian, 

 and that it migrated eastward, by land connections then 

 existing, to North America, here to continue its evolution. 

 The succeeding genera in North America are increasingly 

 numerous, while in Europe the line is disconnected, its occa- 

 sional representatives probably being derived from those that 

 had found their way westward from this country. 



The American Series starts with the Eohippus, or " Dawn 

 Horse. " It comes from the Lower Eocene of Wyoming and 

 New Mexico, and is much more available for study than the 

 Hyracotherium. It is about the size of our domestic cat, and 

 is like the Hyracotherium except that the fusing of the cusps 

 into crests has progressed and the fourth premolar is beginning 

 to look like a true molar. The hand of this animal has four 

 functional fingers, while the thumb is rudimentary and reduced 

 to a splint. The foot has three functional toes, no trace left 

 of the first toe, and the fifth reduced to a splint. It might l)e 

 explained parenthetically that terms applied to the human 

 digits are used here in describing the front feet in order that 

 we may more readily follow these changes by reference to our 

 own fingers. 



The second in the series is the Orohippus or " Mountain 

 Horse." It is found in the Middle Eocene of Wyoming, and 

 in size is somewhat smaller than a fox. Like the liohippus 

 it has four functional fingers and three functional toes, but 

 the splint of the thumb has disappeared, as has the splint of 

 the fifth toe. The radius and ulna and tibia and fibula are 

 still distinct. The crests on the molars are clearer than in the 

 preceding stage and the last premolar is like the true molars, 



