HISTORY OF THE ARTIODACTYLA 397 



and to observe in how many respects they have followed a 

 parallel course, and how nearly \Poebrotherium occupied the 

 same position with reference to the modern camels and llamas 

 as \Mesohippus did to the Recent horses ; but such a com- 

 parison would involve too many technicalities to be profitably 

 undertaken here. Suffice it to say that in many details there 

 was a genuine parallelism in the progress of these two widely 

 separated families from a polj^dactyl ancestry towards an ex- 

 treme of digital reduction, ending in the horses in the single- 

 toed and in the camels in the two-toed foot. The members of 

 the two series kept nearly equal pace in their slow progress, 

 with the camels a little in advance, since they were the first 

 to attain the modern state of development in the height of the 

 teeth and the structure of the feet, though eventually the horses 

 surpassed them in both respects. 



In the upper Eocene (Uinta stage) there were at least two 

 kinds of camels, the time-relations of which to each other are 

 not known, that is, whether they were contemporary or suc- 

 cessive. The best-known genus, \Protylopus, may perhaps 

 not be in the direct line of camel descent, but it so nearly 

 represents the proper ancestral stage that, for all practical 

 purposes, it will serve nearly as well. It was a much smaller 

 animal than the smallest of the Wliite River species, and was 

 hardly larger than a ''jack-rabbit." The teeth of each jaw 

 were in continuous series and the canines were but slightly 

 longer than the incisors ; the premolars had less antero-pos- 

 terior extension than in ^Poebrotherium, and all the molars 

 above and below were very low-crowned. The skull was 

 almost a miniature copy of that of \Poebrotherium, but more 

 primitive in a number of details, the most important of which 

 was that the tympanic bullae were much smaller and hollow, 

 not filled with spongy bone. The neck, concerning which it 

 would be very desirable to have information, is almost the 

 only part of the skeleton that is not known. The fore limb 

 was considerably shorter than the hind, making the back slope 



