356 MAMMALIA. 



little inflated, but small. The dentition is complete, without any diastema. 

 The upper molars exhibit only the four ordinary crescentic cusps, without 

 any intermediate tubercle. The upper canine is somewhat enlarged, 

 pointed, and triangular in section, worn on its hinder face. The upper 

 incisors are relatively small, and nearly uniform. The hindermost lower 

 molar is produced into a third lobe posteriorly. The foremost premolar 

 is quite canine-like in shape, though smaller than the upper canine against 

 which its anterior face works. The lower canine resembles the adjoining 

 incisors, which are about as small and regular as those of the upper jaw. 

 The cervical vertebrae are slightly opisthocrelous ; there are 13 true dorsal 

 vertebrae with ribs; and the caudals of the long tail are known to have 

 been more than 17 in number. The scapula is elongate- triangular in 

 shape ; the humerus is comparatively stout ; the radius and ulna are 

 complete and separate ; while the manus, in addition to the four 

 functional toes, bears a much-reduced though complete digit no. i. 

 In the hind limb the tibia is shorter than the femur, and the fibula 

 is comparatively slender though separate. Among the tarsal bones, 

 only the ecto- and meso-cuneiforms are fused together ; and there is 

 no trace of digit no. i. in the pes. The typical species is Oreodon 

 culbertsoni from the Lower Miocene (White River Formation) of Dakota 

 and Wyoming, with the skull about 0'2 m. in length. Other forms are 

 also recorded from the same horizon in Dakota, Nebraska, Colorado, 

 and Wyoming. 



Agriochcerus (fig. 204). Also known by the complete skeleton. The 

 skull differs from that of Oreodon in the absence of an antorbital pit and 

 the incompleteness of the postorbital bar. The incisors have disappeared 

 from the upper jaw, leaving the small premaxillse toothless ; there is a 

 wide diastema in the upper jaw between the canine and premolars, and a 

 somewhat shorter diastema in the lower jaw between the caniniform pm. 1 

 and pm. 2 ; while the hindermost premolar in both jaws is as complex as 

 the molars. The skeleton is generally similar to that of Oreodon, but the 

 ungual phalanges on both feet are relatively large and claw-shaped rather 

 than hoof-shaped. The typical and best-known species is Agriochcerus 

 latifrons (fig. 204), an animal somewhat more than a metre in length, not 

 including the tail. It occurs in the Lower Miocene (White River Forma- 

 tion) of Dakota, and other forms are met with in the overlying John Day 

 Series. 



Among the artiodactyls contemporary with the Oreodon- 

 tidse in the North American area, it is interesting to be able 

 to recognize distinctly the direct ancestors of the existing 

 Camelidae, or camels and llamas. These animals, as is well 

 known, are unique among the ruminants in several respects 

 and are placed by zoologists in a distinct group, named TYLO- 



