490 



MAMMALS. PHYLUM CHORDATA 



but the blastocyst (p. 666) stops its development and remains 

 free in the uterus throughout the winter. In the spring, develop- 

 ment continues, and a placenta is formed in the usual way. 



PERISSODACTYLA 



No members of this order are wild in Britain to-day, and as 

 it includes only the horses, rhinoceroses and tapirs, it must be 



■h 



Fig. 378. — A side view of 

 the lower part of a 

 pony's foreleg. — From 

 Thomson. 



h.. Distal end of humerus ; m., 

 olecranon process of ulna ; r., 

 radius ; sc, scaphoid ; /., lunar ;' 

 c, cuneiform ; »«., os magnum ; 

 un., unciform ; p., pisiform ; 

 mc.^, splint of fourth metacar- 

 pal ; WC.3, third metacarpal ; 

 s., sesamoid ; i, 2, 3, phalanges 

 of third digit. 



Fig. 379. — A side view of 

 the ankle and foot of a 

 horse. — From Thomson. 



a., Astragalus ; c, calcaneum ; 

 n., navicular ; ex., external 

 cuneiform ; cub., cuboid ; W/.3, 

 third metatarsal ; mt.^, splint 

 of fourth metatarsal ; s., sesa- 

 moid ; ph.i-^, phalanges of 

 third digit. 



