Studies for Students. 



THE DEVELOPMENT AND GEOLOGICAL RELATIONS 

 OF THE VERTEBRATES. 



V. MAM MALI A. — ( Continued?) 



Hippoidea, Equidae. — The phylogeny of the horses has 

 been made out by a study of the gradual changes which suc- 

 ceeded one another in the development of the modern form of 

 the teeth and the foot structure. A nearly perfect series of these 

 changes is known, and besides the direct line of the develop- 

 ment of the horse, there have been demonstrated several side 

 lines, all of which have become extinct. The earliest form that 

 can be definitely stated as belonging to the line of the horses is 

 Hyracotherium, from the Lower Eocene of the continent of Europe, 

 England, and the United States. It is to be noticed that it does 

 not occur in the lowest Eocene; thus it is not found in the 

 Puerco of the United States, with the Condylarthra, but is found 

 in the Wasatch, Green River, and the Bridger. There are four 

 digits on the front foot and five on the hind foot, the three mid- 

 dle digits being functional on both feet ; the fibula and the ulna 

 of the fore part of the front and hind limbs are fully formed, and 

 show nothing of the reduction that they subsequently undergo. 

 The upper molars are different from the premolars, and are fur- 

 nished with six tubercles, an outer pair and an inner pair, and 

 between these another pair that is smaller. 



Pachynolophus is from practically the same horizons as the 

 Hyracotherium in America and on the continent. It represents a 

 short step in advance ; the ulna and the fibula are smaller and 

 barely reach the end of the radius and the tibia. The two inner 

 tubercles of the teeth are elongated laterally and almost join 

 the two middle ones. This form includes, according to Osborn, 



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