260 THE HORSE AND ITS RELATIVES 



proboscis may have been developed in most or all 

 of the forerunners of the horse-group. 



Against this view it may be urged ti\%\.Hippidium, 

 which probably had a proboscis like Onohippidium 

 as the skull-structure in the two genera is almost 

 identical lacks a preorbital pit. As the various 

 opinions in regard to the function of that pit have 

 been fully discussed in the first chapter, no further 

 reference to the subject is necessary in this place. 



It remains, however, to add that a small species 

 of hipparion of slender build from the Pliocene 

 strata of the Siwalik Hills of Northern India, 

 described in the first half of the nineteenth century 

 by Messrs. Cautley and Falconer as Hippotherium 

 antilopinum {Hippotherium being an alternative 

 name for Hipparion) is now believed to have lost 

 the lateral toes, and has accordingly been referred 

 to a genus by itself, under the name of Hippodactylus. 

 In addition to the typical Hippodactylus antilopinus, 

 there is a second Siwalik species, which has been 

 named H. chisholmi? 



It may be added that in Hipparion, as well 

 as in Merychippus, the terminal bone of the main 

 toe has a cleft in the middle of its lower front 

 border ; this cleft occurring in many of the earlier 

 forerunners of the horse. 



All the foregoing genera may undoubtedly be 

 included in the same family Equida as the 



1 G. Pilgrim, Rec. Geol. Suru. India, vol. xl. p. 67, 1910. 



