STJBOBDER C 



PERISSODACTYLA 



129 



that finally it entirely disappears. Of the metacarpals, the maximum number 

 is four, three are usually present, while in the horse only one, the third 

 metacarpal, is functional. The pollex is always absent ; the fifth digit, when 

 present, remains shorter than the other digits. The metacarpals with their 

 slightly concave proximal articulations force themselves in somewhat between 

 the carpals, and thus exhibit an alternating arrangement. The metapodials 

 vary greatly in length. In general the lengthening of the metacarpals 

 signifies advance and usually goes hand in hand with the reduction of 

 the lateral digits. As the weight of the body is gradually transferred to the 

 single middle digit, there is thus formed on the distal convex articulation of 

 the third metacarpal a distinct median ridge, which prevents a lateral displace- 

 ment of the digit. In the older three-toed or four-toed perissodactyls, these 

 keels are developed only on the posterior side of the articulation. The tri- 

 angular, distally flattened terminal phalanges are encased in hoofs. 



The femur is characterised by a prominent, often remarkably well- 

 developed third trochanter. In the forms in which the median digit is less 

 dominant, the tibia and fibula are complete and separate for their entire length ; 

 in the later Eqiiidae, the fibula is reduced to a short proximal splint bone, 

 and its distal end anchylosed to the tibia. 



Of the tarsal bones, the calcaneum (Fig. 165) has an elongated truncated 

 tuberosity {tuber calcis) with a rugose terminal surface, and a sustentaculum 

 directed inward, which supports the astra- 

 galus, and is furnished with three facets. 

 The astragalus (Fig. 166) has above and in 

 front a deeply grooved pulley -like articulation 

 (/) for the tibia, on the inner and posterior 

 sides three facets for the calcaneum, and on 

 the truncated, either plane or slightly convex 

 distal surface a large facet for the navicular 

 (n) and usually a small one for the cuboid 

 (cub). Between the bones of the proximal 

 and distal series lies a low navicular, which 

 is completely covered by the astragalus and 

 in turn is supported by the three cuneiforms 

 of the distal series. The cuboid is high, 



carries the calcaneum, but likewise always articulates with the astragalus, 

 except in Moropus in which the cuboid does not touch the astragalus ; a 

 typical character of Meniscotherium of the early Tertiary. Fusion of the two 

 inner cuneiforms occurs only in the more specialised Equidae ; otherwise all 

 the tarsals remain distinct. The reduction of the lateral digits is more 

 complete and constant in the hind-foot than in the fore-foot (Figs. 167, 

 173). The hind-foot is therefore either tridactyl or in the later 'Equidae 

 monodactyl. The metatarsals with their usually plane proximal articular 

 surfaces do not push in between the tarsals. 



The perissodactyls are divided into five families as follows : Tapiridae, 

 Ehinoceridae, Equidae, Tit another iidae and Chalicotheriidae. Of these the first 

 four may be traced back to a common ancestral form, which probably was 

 very like the condylarth genus Tetraclaenodon. 



Fig. 165. 



Palaeotherium. 

 Calcaneum. as, p', 

 Facet for the astra- 

 galus ; cub, facet for 

 the cuboid. 



Fig. 166. 



Palaeotherium. 

 Astragalus. 

 t. Trochlea ; n, cub, 

 facet for the navicu- 

 laP'and cuboid ; c, c', 

 surface adjacent to 

 the calcaneum ; a, 

 hollow for the lower 

 end of the tibia. 



VOL. Ill 



