Class 6. Mammalia. Order 5. Ungulata. 503 



related to the Tapirs; it had, however, only three toes on each fore-foot, and 

 the molars were like those of the Rhinoceros. 



2. The Rhinoceros. Both fore and hind limbs symmetrical and three- 

 toed ; the median toe (third) somewhat stronger than the other two (second and 

 fourth) ; digitigrade. i ~E^, c §, p f , m f ; incisors more or less degenerate, 

 no canines, strong plicate molars. Antero-dorsally in the median line of the 

 head one or two horns (cf. p. 471). The skin is thick, inflexible, and veiy 

 sparsely covered with hairs ; the upper lip very mobile. Limited to the warmer 

 parts of Asia and Africa. In Africa there are two species with smooth skin 

 and with two horns {Rh. hicomis and simus) ; in Asia a two-homed species and 

 also a one-homed species with large deep skin folds {Bh. unicornis, etc.). The 

 Woolly Rhinoceros {Bh. tichm-inus), with ossified nasal septum, two 

 horns and an abundant covering of hair, lived in Quartemary times in Central 

 Europe and Siberia, together with the Mammoth. 



3. The Horse Family (Equidx) is characterised by the great develop- 

 ment of the middle (third) as compared with the lateral toes, and by the great 

 length of the metatarsus. Fore and hind limbs similar. They tread upon 

 the hoof. Dental formula complete : i i, c \, p ^, m J. Bony orbit complete 

 [i.e., a process from the frontal has united with a process of the zygoma behind 

 the eye. 



[a) All Horses now living belong to the genus Equus. The second 

 and fourth toes ai-e altogether absent, so that each foot has a single toe only, 

 the third, which with the corresponding metacarpal or metatarsal (cannon bone) 

 is extremely well-developed ; the second and fom-th metacai-pals (or metatarsals) 

 are present in the form of long, thin bones (splint bones) on the sides of the 

 caimon bone. The animals tread only on the hoof upon the last phalanx; 

 this hoof surrounds the very small sole of the foot {cf. p. 470, Fig. 382 D). The 

 incisors are characterised by the possession of a large pit partially fiUed with 

 cement (the mark) ; the canines are well developed in the male, rudimentary in 

 the female ; the germ of the fii-st premolar is present in both upper and lower 

 jaws, but usually only develops in the upper ; even there it is radimentary, and 

 generally falls out early ("wolf tooth) ; the other molars of both jaws are of about 

 equal size (broader in the upper than in the lower jaw) ; they have very long 

 crowns and short roots ; the crowns exhibit folds and pits ; these reach to the 

 roots and are filled with cement, which is extraordinarily well- developed here, 

 surrounding the crown with a thick coat ; very soon after the tooth comes into 

 use, streaks of enamel appear upon the grinding surface, and the crown becomes 

 gradually worn away, so that in old Horses it is very short. In contrast to the 

 conditions in the Tapir and Rhinoceros, the lower portion of the ulna (or fibula) 

 is very weak, in part only i-epresented by a ligament. To this genus belong : 

 the Zebras {E. zebra, quagga, Burchelli, and others), with dark transverse 

 stripes; small hoof s ; bovine tail : in South Africa. The Ass {E. asm-Ms), with 

 a black stripe along the middle of the back, and a similar transverse one across 

 the shoulders ; small hoof ; tail bovine : wUd in North Africa. Two allied forms 

 {E. hemionus, the Dsohiggetai, and E. onager, the Kulan, in Asia). The Horse 

 {E. caballus) is usually larger than those just mentioned : it has larger hoofs ; 

 the tail is covered with a large tuft of long hairs : there are naked, horny patches 

 on the skin both of the fore and the hind limbs, the so-caUed chestnuts and 

 ergots (in others, only on the fore limbs) ; native place not definitely known. 



(6) Some of the Quaternary and Pliocene Horses also belong to the genus 

 Equus, and occurred not only in the Old World, but also in the New. Other 

 Pliocene forms belong to the genus Hipparion, a small form which resembles 

 Equus in most respects, but differs from it in that the second and fourth 

 digits were present on all fom' limbs, although only as poorly developed 



