290 MAMMALIA. 



premolars are simpler than the molars. All the vertebral centra 

 exhibit flattened articular ends, and none of the zygapophysial 

 facettes are curved. The humerus differs from that of all other 

 Ungulata (except the Typotheria) and resembles that of some 

 rodents in being pierced by an entepicondylar foramen ; the 

 femur bears a third trochanter. The manus and pes are 

 complete (fig. 164), with five hoofed digits, of which the third 

 is the largest. They are plantigrade or slightly approaching 

 the digitigrade position. The two series of carpal bones do 

 not alternate, but are directly opposed ; and an os centrale is 

 sometimes distinct. In the tarsus the astragalus and calcaneum 

 articulate respectively with the navicular and cuboid. The 

 astragalus is shaped almost as in the Carnivora, but its upper 

 surface, for articulation with the distal end of the tibia and 

 fibula, is only slightly sinuous, not tongued or grooved. The 

 calcaneum is scarcely if at all in contact with the fibula. 

 Hyracops and Phenacodus may be mentioned as typical genera. 



Phenacodus (fig. 165). The head is remarkably small. The skull is 

 elongated, with the interorbital space wide and flat, the nasal bones ab- 

 breviated, and the premaxilla somewhat produced. The brain-cavity is 

 relatively very small and its case is surmounted by a short but elevated 

 sagittal crest. There is no postorbital process either on the frontal or 

 jugal, and the zygomatic arch is not expanded behind. The mandibular 

 rami are not fused together at the symphysis. The canine teeth are not 

 much enlarged, and the anterior premolars are a little spaced. The fore- 

 most premolar in each jaw is simple, with only one root. Pm. 3 and 4 in 

 the upper jaw have an inner lobe and two outer cusps ; while each of the 

 upper molars bears six tubercular cusps, two being internal, two inter- 

 mediate, and two external. Each of the lower true molars bears two pairs 

 of tubercular cusps, with an intermediate odd cusp behind. The axis 

 exhibits a peg-shaped odontoid process and bears a large neural spine. 

 The thoracic or dorsal vertebrae are 14 or 15 in number, the lumbars 6 or 

 7, and the caudals very numerous. There is no clavicle. The scapula 

 exhibits a short coracoid process. The ilium is not much expanded, but 

 the hind limb is relatively very large. Both feet are pentadactyl, with 

 the axis passing through digit no. in, as in the Perissodactyla ; and the 

 metacarpal and metatarsal (or metapodial) bones are keeled distally. The 

 animal may, indeed, have been as completely digitigrade as the tapir. 

 Two species, Phenacodus primcevus (fig. 165) and P. wortmani, are known 

 by nearly complete skeletons from the Lower Eocene (Wasatch Formation) 

 of Wyoming, the first about as large as a tapir, the second smaller. 

 Remains of closely allied genera occur in the basal Eocene or Puerco 



