PHYLOGENY. 147 



way from the perissodactylous genus Hyracotherium 

 Owen. Phenacodontidae are indicated as the ances- 

 tors of all the Ungulata by their character as "buno- 

 dont pentadactyle plantigrades," characters in which 

 they agree with the ancestors of all placental mam- 

 mals. That they are not the ancestors of all the latter 

 is shown by the fact that their molar type is quadritu- 

 bercular; but one has to go backwards but a short 

 distance in time to the Puerco epoch, to find their tri- 

 tubercular ancestors. Between these and Phenaco- 

 dus, comes the quadritubercular genus Euprotogonia 

 Cope, of the Puerco, which has simpler premolar teeth. 

 Between Phenacodus and Hyracotherium there is 

 room for two or more genera with fully facetted car- 

 pals and tarsals, longer feet, and a rudimental first toe 

 on the anterior foot, and first and fifth toes on the hind 

 foot. In Hyracotherium these digits have disappeared. 

 Further, in Hyracotherium the internal cusps of the 

 molars are more or less connected with the external 

 by low and indistinct ridges, which in the superior 

 molars include the small intermediate tubercles or 

 eonules. Thus is the lophodont dentition foreshad- 

 owed. Hyracotherium was a contemporary of Phe- 

 nacodus and continued later in Eocene time. Some 

 of its forms developed an increased complexity of the 

 last premolars in both jaws, forming the genus Pliolo- 

 phus, and foreshadowing the development of molar- 

 like premolars, which is so characteristic of the later 

 members of the horse line. In the genus Epihippus 

 Marsh, of one epoch later in time (the Uinta), two 

 such premolars are developed in each jaw. We have 

 seen very short interspaces next the canine teeth in 

 Phenacodus, and these have become longer in Hyra- 

 cotherium and Epihippus. 



