UNGULATA. 325 



tail is proved to have been of moderate length. The scapula exhibits a 

 larger incurved coracoid process than usual in the Perissodactyla. The 

 limbs are slender. The ulna and radius are separate, and the manus 

 exhibits four complete digits, without any rudiment of no. i. The tibia 

 is relatively stout, and the separate fibula is very slender, though complete. 

 There are only three complete digits in the pes, which is longer than the 

 mauus, but a rudiment of metatarsal no. v is sometimes present. The 

 metapodial bones are keeled distally. Hyracotherium was first known by 

 an imperfect skull and dentition, and then by the greater part of the skele- 

 ton, from the London Clay (Lower Eocene) of the London Basin (H. 

 leporinum). Remains of the same and allied genera (e.g., Pachynolophus) 

 are also known from the Lower and Upper Eocene of France and Switzer- 

 land. The most satisfactory skeletons hitherto obtained, however, are 

 those from the Lower Eocene (Wasatch and Wind River Formations) of 

 North America (fig. 184). Several species are distinguished by the 

 dentition, none much exceeding a fox in size. Orohippus (fig. 182) is a 

 well-known closely allied genus from the Middle Eocene (Bridger Forma- 

 tion) of Wyoming. 



The next group of perissodactyls, considered both in rela- 

 tion to its geological age and its degree of specialization, is 

 that of the Palaeotheriidae. thus named from the typical 

 genus Palceotherium, which is found only in the Upper Eocene 

 of Europe, but is succeeded by other forms occurring both in 

 Europe and America. The dentition is complete, or wants 

 only pm. 1, and the grinding teeth are brachyodont, without 

 any trace of cement in the hollows of the crown. Each upper 

 molar bears two oblique ridges united at the outer end by a 

 W-shaped external ridge or wall ; while each lower molar 

 comprises two crescentic ridges. The hinder premolars in 

 both jaws are almost or quite as complex as the molars. The 

 canines are always well developed, though never excessively 

 enlarged; the incisors are simple cutting blades, but in the 

 latest genus, Anchitherium, they begin to exhibit traces of the 

 pit which becomes the " mark " in the horses. There are three 

 complete digits on each foot. 



Palseotherium (figs. 185, 186). The skull in this genus considerably 

 resembles that of an existing tapir, with large freely projecting nasal 

 bones, pointed in front, which may perhaps indicate the original presence 

 of a short proboscis at least in some species. The small orbital cavity is 

 not separated by bone from the temporal fossa, while the parietal region 

 rises into a sharp crest. In the dental series there is a considerable 



