IV. VERTEBRATA: MAAIMALIA, UNGULATA 



567 



The ungulates, which arise from the Condylarthra of the eocene (Phcna- 

 codon), are preeminently herljivorous; the canines are rarely well developed, the 

 molars numerous and adapted to grinding the food, more or less flattened and 

 frequently with folded enamel. The mamma; are inguinal, the uterus bicornu- 

 ate, and the placenta either diffuse or (most ruminants) cotyledonary (fig. 6io). 

 The legs are exclusively locomotor structures and the clavicles are absent. 

 Since the metacarpals and metatarsals are greatly elongate, the wrist and ankle 

 are raised from the ground so that they are frequently confounded with elbow 

 and knee. With this exclu.sively supporting character of the limbs there is a 

 tendency to reduction and fusion of bones. There is a constant increase in the 

 development of radius and tibia to the chief supports of the body, the fibula 

 becoming rudimentary, the ulna being developed sometimes throughout its 

 whole extent, sometimes only in its upper part, and is more or less fused with the 

 radius. The same tendency to simplification prevails in the feet, but is ex- 



je- m 



PiQ^ 6jj^ — Pore feet of ungulates (after Flower). A-C, peri.s5odactyle ; D-F, 

 artiodactyle. A, tapir; B, rhinoceros; C, horse; /), pig; E, deer; F, camel, c, trique- 

 trum ("ulnare); I, lunatum (intermedium); m, capilatum; m^-m^, rudiments of meta- 

 carpals II and V; p, pisiforme; R, radius; s, scaphoid (radiale); td, trapezoid; tm, 

 trapezium; U, ulna; u, hamalum; II-V, digits. 



pressed differently in the odd-toed {perissodacfylc) and even-toed (artiodaclyle) 

 forms. In the Perissodactyla the axis of pressure passes through the middle 

 toe (fig. 615, A-C, III), while the other toes disappear symmetrically around 

 this. Since the first toe is early lost, toe V is next to disappear {B), and then 

 toes II and IV (C), so that at last there remain only the skeleton of the middle 

 toe (horse), the rudiments of toes II and IV persisting as the small splint hones. 

 In the Artiodactyla the axis of pressure falls between toes III and IV (£>), 

 which support the body, are equally developed and frequently fuse, at least so 

 far as the metacarpals are concerned [E, F). The figures D-F show how the 

 other digits disappear, digit I being lost still earlier. Since the weight of the 

 body rests more upon the hind legs than upon the front ones, the former are 

 the first to become modified. Since we are able, with the fossils, to follow in 

 detail the lines of descent of both artiodactyles and perissodactyles, it is certain 

 that these form diverging series, distinct from the beginning. In each series 



