A HISTORY OF SCIENCE 



during the three diversions of the tertiary period were 

 especially numerous in the Rocky Mountain regions, 

 and their remains are well preserved in the old lake 

 basins which then covered so much of that country. 

 The most ancient of these lakes which extended over 

 a considerable part of the present territories of Wyo- 

 ming and Utah remained so long in eocene times that 

 the mud and sand, slowly deposited in it, accumulated 

 to more than a mile in vertical thickness. In these 

 deposits vast numbers of tropical animals were en- 

 tombed, and here the oldest equine remains occur, 

 four species of which have been described. These 

 belong to the genus Orohippus (Marsh), and are all of a 

 diminutive size, hardly bigger than a fox. The skele- 

 tons of these animals resemble that of the horse in 

 many respects, much more indeed than any other ex- 

 isting species, but, instead of the single toe on each 

 foot, so characteristic of all modern equines, the various 

 species of Orohippus had four toes before and three 

 behind, all of which reached the ground. The skull, 

 too, was proportionately shorter, and the orbit was not 

 enclosed behind by a bridge of bone. There were fifty- 

 four teeth in all, and the premolars were larger than 

 the molars. The crowns of these teeth were very short. 

 The canine teeth were developed in both sexes, and the 

 incisors did not have the "mark" which indicates the 

 age of the modern horse. The radius and ulna were 

 separate, and the latter was entire through the whole 

 length. The tibia and fibula were distinct. In the 

 forefoot all the digits except the pollex, or first, were 

 well developed. The third digit is the largest, and its 

 close resemblance to that of the horse is clearly marked. 



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