326 



PRINCIPLES OF ANIMAL BIOLOGY 



the feet diagrammatically. Of the three well developed toes, the middle 

 one (third) was in each foot distinctly larger than the others. The skull 



Fig. 222. — Fossil teeth of ancient horse-like animals, a, tooth of Eohippus with the 

 roots broken; b, tooth of Mesohippus; c, tooth of Merychippus. (Photographed from 

 specimens in the Zoological Laboratory of the University of Michigan.) 



(Fig. 224), except for its increase in size, had not changed strikingly. 

 The crowns of the molar teeth were still low (Fig. 222) and were tubercu- 



FiG. 223. — Skull of Eohippus, about ^q natural size. {From model prepared by Ward's 



Natural Science Establishment.) 



late, that is, provided with cusps on the upper surface, but the cusps 

 were more distinctly united into ridges or crests. 



Fig. 224. — Skull of Mesohippus, about ^fo natural -size. (From photograph of specimen 

 in Museum af Geology, University of Michigan.) 



Merychippus. — The feet of Merychippus were all three-toed (Fig. 

 221), vestiges of the fifth toe being present in some specimens and wanting 

 in others. The lateral toes, however, were high above the ground, 



