300 



LAND MAMMALS IN THE WESTERN HEMISPHERE 



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

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

 {'\Mesohippus) which characterizes the White River, or lowier 

 Ohgocene, 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 



ftG. 152. — The small, browsing, three-toed, short-necked horse (^ Mesohippus bairdi) 

 of the middle White River. Restored from a skeleton in the American Musemn. 



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 



