252 THE HORSE AND ITS RELATIVES 



elongated noses, not improbably forming a kind 

 of short trunk comparable to- that of the saiga 

 antelope. 



In that animal, as well as in its relative the 

 chiru of Tibet, the increased size of the nasal 

 chamber has been brought about by a shortening, 

 instead of an elongation, of the nasal bones, but it 

 is probable that in these two antelopes and in the 

 hippidium the purpose of the modification is the 

 same. It has been supposed that in the case of 

 the chiru the large size of the nasal chamber is an 

 adaptation to the respiratory needs of an animal 

 living at a very high elevation ; but in the case of 

 the saiga such an explanation cannot hold good ; 

 and the real explanation in all three cases may 

 perhaps be found in a special adaptation to a desert 

 life, the long nose serving as a filter to prevent 

 particles of sand reaching the organ of smell. 



As regards the rest of its skeleton, Hippidium 

 is remarkable for its short and stout limbs ; this 

 being chiefly due to the excessive shortness of the 

 cannon-bones, which are also unusually wide, and 

 the great stoutness of the splint-bones. Each limb 

 terminates in a single toe. These short limbs, 

 coupled with the huge, unwieldy head, indicate that 

 the hippidium had less speed than ordinary ponies. 

 There are only five ribless trunk, or lumbar, verte- 

 brae, as in the Arab horse. 



The skull of Hippidium shows no marked 



