272 THE STRUCTURAL EVIDEN^OE OF EVOLUTION. 



smooth convexity to the hollow of the applied heel-bone behind. 

 No such astragalus has ever been found in the Lower Eocene forma- 

 tions of America ; animals bearing it in a less perfect stage appear 

 in the next higher period, the Miocene ; but it is not until the 

 Pliocene and modern times that they abound. In the Hippopota- 

 mus foot, we have an example of the less perfect astragalus of this 

 type of animals. The pulley surfaces 

 are flatter and less deeply grooved. 



In the horse, the upper surface 

 of the astragalus forms as perfect 





PERO CU lONSUS 



. for 

 PEPONEUS BPEVIS 



AB DIN. DIG- 



Fig. 49. — Anterior foot of Homo sapiens from 

 above, one third natural size. From Allen's 

 Anatomy. 



Fig. 50.— Posterior foot of do. 

 from above, one third natural 

 size. From Allen's Anatomy. 



a joint as in ruminants, but the lower and hinder faces present 

 the flattened surfaces which belong to the many-toed Mammalia. 

 The lower face especially is mainly occupied by one large facet, in- 

 stead of the two-faced pulley of the ruminants. In the rhinoceros 

 a later facet is more distinct, while in the Miocene Menodus the 

 second facet is larger, resembling, except in the still convex tibial 

 articulation, the structure of that of the primitive Corijphodon. 



