342 MAMMALIA. 



canines are large, and the three anterior premolars are laterally compressed 

 to a sharp edge ; while the fourth premolar above (fig. 196 A) has a supple- 

 mentary inner cone. Each upper molar (fig. 196 A) has a conspicuous 

 bounding basal ridge or cingulum and bears six conical cusps in two 

 transverse series ; but in the second molar the posterior inner cusp is 

 relatively small, while in the third molar it is quite rudimentary. Each 

 of the lower molars (fig. 196 B) bears four conical cusps regularly arranged 

 in two transverse pairs, and the hindermost or third molar is produced 

 backwards into a small third lobe or talon. The cervical vertebrae are 

 short, and exhibit oblique ends, thus indicating a curved neck ; the 

 hinder trunk-vertebrae are keeled below. There seem to have been five 

 digits in each foot, but only four can have been functional. The typical 

 species is Homacodon vagans, known by the greater part of the skeleton 

 from the Middle Eocene (Bridger Formation) of Wyoming. Teeth and 

 astragali from the Lower Eocene of Wyoming and New Mexico have been 

 provisionally ascribed to the same genus. Cebochcerus is an allied genus, 

 from the Upper Eocene of France and Switzerland, and from the Middle 

 Miocene of Steinheim, Wiirtemberg, known only by portions of the skull 

 and the dentition. 



FIG. 196. 



Homacodon vagans ; right upper premolars 2 4, molars 1 3 (A) and lower 

 premolars 3, 4, molars 1 3 (B), twice nat. size. M. Eocene (Bridger For- 

 mation) ; Wyoming. (After Marsh. ) 



Choeropotamus. A larger animal, the mandible of the typical species, 

 C. parisiensis, measuring about O2 m. in length. The dental series is 

 complete, except for the loss of pm. 1 in the lower jaw. Each upper molar 

 is quadrate in form, with a conspicuous basal cingulum, and the conical 

 cusps acutely pointed ; the median cusps are small and irregular compared 

 with the outer and inner pairs. The hindermost upper premolar has one 

 inner cusp, while pm. 3 bears a small inner ridge representing the latter. 

 The lower molars exhibit small intermediate cusps, irregular like those of 

 the opposing teeth ; while lower m. 3 is produced backwards into a large 

 third lobe or talon. The canines are well-developed. C. parisiensis is 

 known only by the jaws and dentition from the Upper Eocene of the Isle 

 of Wight and France. 



