270 THE STRUCTURAL EVIDENCE OF EVOLUTIOK 



ized type to the extreme specializations observed in the horse and 

 the ox. 



The modifications are as follows : The hind foot is composed 

 of two rows of tarsal bones, of which the second is followed by 

 the long metatarsal bones, from which the bones of the toes origi- 

 nate. The second segment of the hind leg is composed of two 

 boues, tibia and fibula, which in the salamander, etc., have a sub- 

 equal union with the foot. In some multidigitates, as the genus 

 Corypliodon, both these bones articulate with the 

 two bones of the first row of the tarsus, and one 

 (fibula) is the smaller of the two. In many higher 

 forms they articulate with but one of these tar- 

 sal bones, viz., the astragalus, with which they 

 form a perfect hinge joint ; the other tarsal bone 

 of the first row is the 

 calcaneum or heel-bone. 

 In Coryjjliodon the as- 

 tragalus and the applied 

 leg-bone (tibia) are near- 

 ly flat, offering an ex- 

 tremely imperfect hinge 

 for the foot, and the 

 heel-bone (calcaneum) 

 is exceedingly short. 

 The animal plainly 

 walked on the entire 

 sole of the foot, and 

 must have had an awk- 

 ward gait, from the Fig. 47.— Left anterior 

 slio'ht power of flexins^ ^ooX.oi Phenacodus primal- foot of PTienacodusprimos,- 



the ankle-joint. From ""''^^"^- '^''- ^^"^^-^ t^^«, K nat. size. (Orig.) 

 this point to the horse on one side, and to the ox on the other, we 

 have a line of succession of intermediate forms. And before de- 

 scribing them, I may state that the Corypliodon is one of the old- 

 est known Mammalia, its remains having been found in the Lower 

 Eocene Tertiary of New Mexico and Wyoming, while the ox and 

 horse are extremely modern animals, their advent on the earth 

 having preceded that of man by but one geological period. 



The most perfect ankle-joint is that of the ruminating animals. 

 The astragalus presents a deeply grooved segment of a pulley ; an 

 angulated pulley, face downward to the rest of the foot, and a 



Fig. 48. — Left posterior 



