178 R. S. Lull— The Evolution of the Horse Fam ily. 



12 



Fig. 12. 



Milk and b, 



of its ancestors, while in the adult state the teeth are long- 

 crowned and cemented as in its successors. The adult teeth 

 are not very long, but are of the columnar or prismatic 



type and in some instances show a 

 fair degree of complexity in the 

 enamel pattern. Merychippus is also 

 the first horse to complete the hinder 

 border of the orbit by sending down- 

 ward a bony bar' to join the zygo- 

 matic arch.. This bar is represented 

 by a process in its predecessors. The 

 senus ranged from Texas to Montana 

 and Oregon, the Yale collection con- 

 taining specimens from the Loup 

 Fork deposits along the Niobrara 

 River in .Nebraska. 



Protohipims of the Loup Fork 



permanent premolar teeth of bedg ig witfa difficulty distinguished 

 Meruchippus insiqnis. Iwo- „ -.. 7 . ", , ° 



thirds natural size. (Original.) ™m Merychvppus, but. has gone 



farther in that both milk and per- 

 manent teeth are fully cemented. The teeth are lengthened 

 and are sharply curved, owing to the shallow depth of the 

 jaws. Protohippus was still three-toed, but as far as known 

 the vestiges of digits one and five in the hand have now 

 entirely disappeared. This animal was about thirty- six inches 

 in height at the shoulder and its distribution is similar to that 

 of Merychipjms. 



Pliohijppus is found in the Upper Loup Fork beds and 

 ranges up into the Pliocene. It may have been a one-toed 

 horse ; for this we have Professor Marsh's authority, though 

 later writers are not inclined to accept the evidence on this 

 point as final. Certain it is that in the type specimen of Plio- 

 hippus pernix at Yale the lateral toes must have been extremely 

 small, as the splint bones are not much more developed than 

 in the modern horse. Unfortunately the lower ends of all the 

 splints are missing, so that one cannot be sure whether or not 

 they bore an articular extremity. The teeth of Pliohijppus 

 were larger than those of Protohippus, and in some instances 

 still more sharply curved, but the enamel pattern was simpler, 

 with large cement areas. 



Pliohippus is the largest of the Loup Fork horses, being 

 forty-eight inches at the withers as compared with sixty-four 

 in a large modern horse. It may be in the direct line of 

 descent to Equus, but of this we cannot be sure, as all the links 

 in the chain of descent have not yet been found. Its geo- 

 graphical range covered the western United States, the Yale 

 Museum specimens including the types of Pliohippus pernix, 

 P. robustus, and P. gracilis, coming from Nebraska and 



