CHORDATA — VERTEBRATA — MAMMALS 



395 



m.ca. 



Equus 



Merychippus 



Mesohippus Eohippus 



Fig. 175. — Evolution of the horse. III. The left fore foot (viewed from the left 

 side) of the one-toed Upper Pliocene, Pleistocene and modern horse, Equus, the 

 three-toed Miocene horse Merychippus, the three-toed Oligocene horse, Mesohip- 

 pus and the four-toed Eocene horse, Eohippus. All are drawn to same scale. 

 According to Scott (see Fig. 172) the toes should be almost horizontal, ca., carpal 

 (wrist) bones; cu., cuneiform; /., lunar; m.ca., Ill, IV and V, metacarpal three, 

 four and five; mg., magnum; ph. i, 2 and 3, proximal, middle and terminal 

 (ungual) phalanges ; pi., pisiform, — the ulnar sesamoid bone developed in the 

 tendon of the flexor muscle ; scs., the sesamoid bone developed in the tendon behind 

 the junction of the metacarpal and phalanges; un., unciform; ///, IV, V, the 

 digits. (After Matthew.) 



