XIII 



PHYLUM CHORDATA 



527 



never developed. In the Perissodactyla the third digit is sym- 

 metrical in itself. In the Rhinoceros and Tapirs the second and 

 fourth digits are also completely developed ; but in the Horses 

 (Fig. 1175) they are represented only by splint-like vestiges of 



FIG. 1175. Dorsal surface of 

 right tarsus of Horse (Equus 

 caballus). a. astragalus ; c. 

 calcaneum ; cb. cuboid ; c. 

 united meso- and ento- 

 cuneiform ; c 3 . ecto-cunei- 

 form ; n. navicular ; mil, 

 IV, vestigial second and 

 fourth metatarsals ; ///, 

 third ruetatarsal. (After 

 Flower.) 



FIG. 1176. Dorsal sur- 

 face of right tarsus of 

 Red Deer (Cervus 

 elaphus). a. astragalus ; 

 c. calcaneum ; cb. cu- 

 boid ; c a . conjoined 

 ecto- and meso-cunei- 

 form ; mill, mlV, 

 third and fourth meta- 

 tarsals ; n. navicular. 

 (After Flower.) 



>nI7~ 



FIG. 1177. Dorsal sur- 

 face of right tarsus of 

 Pig (Sus scrofa). a. 

 astragalus ; c. calcan- 

 eum ; cb. cuboid ; c 3 . 

 ecto-cuneiform ; c 2 . 

 meso-cuneiform ; mil 

 V, metatarsals ; n. 

 navicular. (After 



Flower.) 



their metatarsals, the metatarsal of the third digit forming an 

 elongated " cannon bone," like the metacarpal of the third digit 

 of the manus. In the Rhinoceroses and Tapirs all the usual 

 tarsal bones are present ; in the Horses the ento-cimeiform and 

 meso-cuneiform are united. In the Artiodactyles the third and 

 fourth digits form a symmetrical pair as in the manus ; and in 

 the Ruminants (Fig. 1176) their metatarsals unite to form a cannon 

 bone. In most Ruminants there are no vestiges of the second 

 and fifth digits. In the Pigs (Fig. 1177) all the tarsal bones are 

 present. In most Ruminants the cuboid and navicular are united ; 

 in the Camels these bones are distinct, but the ento-cuneiform is 

 wanting. 



In the Hyracoidea the scapular is triangular, like that of the 

 Ungulata vera, and the spine is moderately developed, most 

 prominent in the middle. There is a large supra-trochlear fora- 

 men. The radius and ulna are complete, but often ankylosed. 



K K'2 



