298 



VERTEBRATE SKELETON 



third metacarpal. Usually the distal phalanx of each persisting digit is broad- 

 ened to support the hoof. 



The femur, longer than the tibia in tapirs, always has the third trochanter, 

 and a nutrient canal enters the bone at about the middle of the posterior side. 

 Rhinocerotidae and Tapiridae have separate crural bones, but in horses the fibula 

 remains only proximally, fused with the upper end of the tibia. The tarsal 



Fig. 321. — Feet of Perissodactyls: A, AA, fore and hind feet of Tapirus; B, BB, 

 of Rhinoceros; C, CC, of Equus (Butschli, '10). c'"', cuneiformia; ca, calcaneum; cb, 

 cuboid; cp, capitatum; F, femur; /, fiibula; H, humerus; h, hamatum; /, lunatum; n, 

 naviculare; o, olecranon; p, pisiforme; r, radius; t, tibia; ta, talus; id, trapezoid; tm, 

 trapezium; tr, triquetrum; u, ulna; I-V, metacarpals and metatarsals. 



bones are in two rows, the talus with two unequal articular faces. The feet are 

 never plantigrade, the third toe always the largest. Tapirs and Aceratherium 

 have five toes arranged symmetrically about 3; others have the lateral digits 

 more elevated, i being often lost, followed in modern horses by 2 and 4, traces of 

 them being the spHnt bones fused with the cannon bone. 



