394 LAND MAMMALS IN THE WESTERN HEMISPHERE 



cursorial creature and had a very exceptional feature in its 

 dentition in the apparent presence of ten lower incisors, five on 

 each side, the canine and first premolar having assumed the form 

 and functions of the incisors ; the molars were low-crowned. 

 The head was rather small and rounded, the neck long and light, 

 the limbs and feet elongate and excessively slender. The feet 

 had two digits each, which were separate, not forming a cannon- 

 bone, and the hoofs were narrow, pointed and deer-like. These 

 delicate and graceful little animals had but a brief career, which 

 seems to have reached its close in the lower Miocene. Perhaps 

 their complete defencelessness made it impossible for them to 

 maintain themselves against their enemies, despite their evi- 

 dent capacity for swift running. 



The camels of the upper Oligocene (John Day) are still 

 incompletely known, but appear all to have belonged to the 

 series of grazers which led up to the modern genera. Future 

 discovery may bring to light in the John Day earlier members 

 of the tgiraffe-camel series, of which a possible member is 

 found in the uppermost substage of the White River, or perhaps 

 both phyla united in the upper Oligocene, a question which 

 remains to be determined. At all events, in the middle sub- 

 stage of the White River, or lower Oligocene, there is no evi- 

 dence of more than a single phylum, from which the others 

 were almost certainly derived, branching off from the main 

 stem at different levels. First was given off the branch of the 

 tgiraffe-camels, then (or perhaps even earlier) that of the 

 little tgazelle-camels, and, finally, the main stem bifurcated 

 into the two phyla of the llamas and the true camels. The 

 point of origin of the tgazelle-camels is still uncertain. 



The typical White River genus {\Poebrotherium) included 

 a series of species which increased in size from the earlier to the 

 later portions of the stage, but showed no such structural changes 

 as to call for special notice. The larger of these species was 

 somewhat taller than a sheep, but of much lighter proportions, 

 with small, pointed head, long neck and body and long, very 



