jR. S. Lull — The Evolution of the Horse Family. 177 



MIOCENE PERIOD. 



This was a time of continental elevation and great expan- 

 sion of our western prairies and a consequent diminution of 

 the forest-clad areas. Many mammals otherwise well fitted 

 for survival, such as the titanotheres whose remains are very 

 numerous in the Oligocene beds, were unable to meet the new 

 conditions because of their very perfect adaptation to softer 

 herbage, and thus became extinct. This was also true of cer- 

 tain horses, such as Hypohippus, but the great majority were 

 more plastic and in consequence underwent a remarkable 

 development, during this period reaching the culmination in 

 numbers and kinds. 



Hypohippus has already been alluded to as one which 

 became extinct during the Miocene, leaving no descendants. 

 It is generally referred to as the " forest horse " from the 

 broad low-crowned teeth fitted only for browsing upon succu- 

 lent herbage, and from the character of the spreading three- 

 toed feet in which the lateral digits still touch the ground. 

 This would imply an animal fitted for soft ground rather than 

 for speed over the dry prairie soil. The reindeer, whose home 

 is on the mossy tundras, has a broad spreading foot with large 

 lateral toes, while in the fleet-footed prong-horn antelope of 

 our western plains the lateral hoofs have entirely disappeared. 



In the hand of Hypohippus vestiges of digits one and five 

 may still be seen as small nodules of bone at the back of the 

 wrist, This would imply the descent of this genus from some 

 undiscovered Oligocene ancestor, for in no known instance do 

 we find traces of the first digit in a horse of that period. 

 Hypohippus was a comparatively large horse for its time, 

 being forty inches at the withers. It is an admirable example 

 of arrested evolution. There are few specimens of this form 

 in the Yale collection ; hence it is represented in the series 

 by a fore foot and lower jaw and by casts of the skull, jaws, 

 and feet of a mounted specimen in the American Museum. 

 Hypohippxis has been found in the Loup Fork beds of Mon- 

 tana and South Dakota.' 



Merychippus is of especial interest and is in the direct line 

 of descent, through some of its species giving rise to all sub- 

 sequent Equidse. It is three-toed, in some instances with ves- 

 tiges of the outermost digits of the hand. Digits two and 

 four vary somewhat in development in the different species, 

 though never reaching the ground, so that the feet are func- 

 tionally one-toed. 



It is in the teeth that the greatest interest lies, for herein 

 Merychippus is midway in the course of evolution, in the 

 young condition having the short-crowned uncemented teeth 



Am. Jour. Sci. — Fourth Series, Vol. XXIII, No. 135.— March, 1907. 

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