1 64 STUDIES FOR STUDENTS 



a large number of genera from both the old and the new worlds 

 that have been described from separate teeth and fragmental 

 bones. 



Epihippus from the Uinta and Bridger and Propaleotherium 

 from the Middle Eocene of the Paris Basin, are very close to 

 the Pachynolophus, but the inner tubercles of the upper molars 

 are more perfectly joined to the middle tubercles, so that there 

 is a short ridge on both the anterior and the posterior portion of 

 the inner side of the tooth. 



Mesoliippus from the Oligocene of the United States, White 

 River, and Paleotherium from the Upper Eocene and the Oligo- 

 cene of France, are the best representatives of the next step. 

 In these forms there are only three digits on the front and hind 

 feet, the fourth digit on the front foot being reduced to a mere 

 splint bone with no trace of terminal phalanges. The ulna and 

 the fibula are greatly reduced and scarcely reach to the distal ends 

 of the radius and the tibia. The connections between the two 

 inner sets of tubercles of the upper molars have developed into 

 strong cross ridges which extend well out to the outer border of 

 the tooth and nearly touch the anterior edges of the correspond- 

 ing tubercles. 



Anchitherium is now regarded as little off the main line of 

 descent of the true horses, but as it represents very closely the 

 succeeding stage that is properly indicated by a poorly known 

 form, Desmatippiis, from the Deep Riverbeds of Oregon, it may 

 properly be described here. 



A?ichitherium is one of the most common forms of the Upper 

 and the Middle Miocene of the continent of Europe and the 

 United States. The fourth digit of the front foot is reduced to 

 a mere ossicle of bone at the upper end of the third digit. The 

 distal ends of the ulna and the fibula have entirely disappeared, 

 so that the bone ends free in the middle part. The teeth show 

 the cross ridges of the upper molars extending out to the outer 

 tubercles and joined with them. One of the most important 

 changes of the series that developed the horse appears in this 

 form; the enamel on the upper portion of the cross ridges dis- 



