304 LAND MAMMALS IN THE WESTERN HEMISPHERE 



neck was very short, the body long, with curved or arched back, 

 the hmbs and feet short, and the hind hmb much longer than 

 the fore, making the relative proportions of the various parts of 

 the skeleton very different from what they afterwards became. 

 Reviewing this marvellous history of steady and long-con- 

 tinued change, beginning with the most ancient genus, \Eohip- 

 pus, the following modifications may be noted : 



(1) There was a nearly constant, if somewhat fluctuating, 

 increase in size, leading by slow gradations from the diminutive 

 horses of the lower Eocene to the great animals of the Pleis- 

 tocene. 



(2) The molar teeth, originally made up of conical cusps, 

 changed to a highly complex pattern of crests and ridges, and 

 the premolars, one by one, assumed the size and pattern of the 

 molars ; the low-crowned, rooted and cement-free teeth, fitted 

 only for browsing, became very high-crowned, prismatic and 

 cement-covered, admirably adapted to grazing. Beginning 

 in the upper incisors of the White River ^Mesohippus, the 

 ''mark" became established as an enamel-lined pit, growing in 

 depth as the teeth increased their length. 



(3) The face grew relatively longer, the eye-socket being 

 shifted behind the teeth and becoming completely encircled 

 in bone, and the jaws were greatly increased in depth to ac- 

 commodate the very long teeth. 



(4) The short neck was greatly elongated and the individual 

 vertebrae modified so as to give flexibility with no loss of 

 strength. The primitive peg-like odontoid process of the 

 axis became first semicylindrical and then spout-shaped. 



(5) The arched back was straightened and the neural spines, 

 especially of the anterior dorsals, elongated. 



(6) The limbs grew relatively much longer ; the bones of the 

 fore-arm and lower leg were fused together, the one on the inner 

 side (radius and tibia) enlarging to carry the entire weight 

 and the external one (ulna and fibula) becoming more or less 

 atrophied. 



