300 



LAND MAMMALS IN THE WESTERN HEMISPHERE 



which was the probable ancestor of '\Hypohippus and the other 

 non-progressive types of the Miocene and Pliocene. The genus 

 (■\Mesohippus) which characterizes the White River, or lower 

 Oligocene, was a group of species of different sizes, becoming 

 smaller as we go back in time, the commonest one being con- 

 siderably smaller than a sheep and differing more or less in all 



Fig. 152, 

 of the 



— The small, browsing, three-toed, short-neckc(i hdisc [i Mc.sohippus bairdi) 

 middle White River. Restored from a skeleton in the American Museiun. 



its parts from the horses of the upper Miocene and all subse- 

 quent formations. The teeth were very low-crowned and 

 fitted only for the mastication of soft vegetable tissue ; but it is 

 of particular interest to observe the beginnings of the ''mark" 

 in the upper incisors in the form of a low enamel-ridge arising 

 behind the cutting edge of the tooth ; the lower incisors still had 

 the simple chisel-like crowns of the more ancient genera ; all 

 the premolars, except the first, had already acquired the 

 molar-pattern. 



The skull resembled that of a very small modern horse, 

 but with many differences of detail, the most obvious of which 

 is the shallowness of the jaws, for depth was not needed to 



