SUBORDER C 



PERISSODACTYLA 



147 



longer reach the ground. The fifth metacarpal is only indicated by a small 

 vestigial nodule. In Pliohippus, Equus and Hippidium the two lateral meta- 

 carpals finally become reduced to slender tapering splint bones, which in the 

 two last genera reach only just above the middle of the long and stout third 

 metacarpal; the latter alone has three well- developed phalanges, the last 

 of which is encased in a large hoof. The fifth metacarpal has completely 

 disappeared. 



The tarsus and hind foot (Fig. 192) undergo a similar modification, so 

 that here also a monodactyl foot with two weak lateral splint bones finally 

 results. The median keels on the distal articular surfaces of the metapodials 

 are at first confined to the posterior side ; in the course of time, however, 

 they become more developed, and in the latest genera divide the entire 

 articular surface into two equal parts. 



The Equidae are divided into four subfamilies : Hyracotheriinae, Palaeo- 

 theriinae, Anchitheriinae and Equinae. 



Subfamily 1. Hyracotheriinae Cope. 



Orbits open behind. Dental formula . 



3.1.4.3. 



c. -, , ^ Cheek teeth very low, with 

 3.1.4.3. ^ 



strong basal cingulum. Premolars usually less compleo: than molars. Superior 



molars with four opposite main cones and two smaller intermediate cusps ; inferior 



molars quadntubercular. The upper 



as well as the lower cusps conical, the 



upper usually united with a straight 



ridge, the lower ones united by a 



\J-shaped ridge. Poadius and ulna 



distinct, about equally well developed. 



Manus tetradactyl, pes tridactyl. 



The Hyracotheriinae are among 

 the oldest and most primitive peris- 

 sodactyls, and in their dentition 

 have in part retained bunodont 

 characters. The conical external 

 cusps of the superior cheek teeth 

 remain either isolated or are joined 

 to a low ectoloph ; the internal and 

 intermediate cones, also, either 

 have no connection with the ex- 

 ternal cusps or are united with 

 them only by an indistinctly de- 



vplnnprl hnnrl or rirlcrp Tlip r>rp Hyraootlierium le2Mrinum Owen. Lower Eocene (London 



Veiopea Oana or rmge. ine pre- ^lay), Heme Bay, Kent, a, imperfect skull, palatal 



molars are nearly always less aspect 3/4. B, Second upper molar. 2/^. C, First lower 



•^ '' molar, ^/i. (After Owen.) 



complex than the molars, usually 



tritubercular. The Hyracotheriinae may be derived from the Condylarthra, 



especially from Tetraclaenodon, and in their turn are the source of the Equinae. 



Remains of this subfamily are found only in the Eocene of Europe and 

 North America, 



Eyracotherium Owen {Pliolophus Owen) (Fig. 193). All cheek teeth 



Fig. 193. 



