ITS ANCESTORS AND RELATIONS. 



31 



that A is not converted into B by merely taking 

 away the digit I. 



It can hardly be supposed that the change took 

 place suddenly from a five-toed to a four-toed fore- 

 foot, and indications have been discovered of inter- 

 mediate forms (EoJiippus of Marsh) in which a rudi- 

 mentary first toe, represented only by the metacar- 

 pal bone, existed, but these have not yet been fully 

 described. 



The hind foot of Hyracotherium presents a still 

 greater modification, both the outer digits, first and 

 fifth, having disappeared. The three middle toes, 



A B C 



FIG. 3. Grinding surface of upper molar tooth (very 

 slightly worn). A, Phenacodus. B, Hyracotherium. 

 C, Anchitlierium. a, antero-external cusp ; &, antero- 

 internal cusp; c, postero-external cusp; d, pos- 

 tero-internal cusp. 



symmetrically disposed to the axis of the third, are 

 alone present. This is the condition of the hind foot 

 of all known true Perissodactyles of the Eocene and 

 Miocene epoch, and of the greater number of those 



