THE GREVY ZEBRA 699 



Dolichohippus is decidedly great. The head in length is quite 

 equal to that of a large draught-horse, an animal having 

 twice the bulk of a Grevy zebra. The lengthening of the 

 skull is due to the production forward of the rostral portion 

 or the part in front of the grinding-teeth, which gives the 

 skull a long diastema or break between the grinding-teeth 

 and the incisors, and also gives the skull a long nasal cavity. 

 The most distinctive feature of the skull is the great width 

 of the occipital crests and their production backward be- 

 yond the occipital condyles. The cheek-teeth are better 

 developed than in the quagga, being larger in proportion. 

 In general shape and size the skull of Dolichohippus re- 

 sembles closely that of horses of Arabian stock, but is dis- 

 tinguishable by the broader occipital crests. The skull of 

 the wild horse, Equus prevalski, is shorter and more like 

 that of the ass and differs more from the long, narrow skull 

 of Dolichohippus than do some domestic races. By some 

 recent writers the Grevy zebra has been considered the 

 least specialized of the living Equidce^ and it has been there- 

 fore assumed that its coloration may be taken as represent- 

 ing that of the ancestral stock. Such a conclusion, however, 

 does not agree well with the extremely long-headed nature 

 of this zebra and the somewhat higher specialization of its 

 dental apparatus. It has been shown by paleontologists 

 that the lengthening of the head in the horse has been a 

 progressive affair which has gone on simultaneously with 

 the gradual elongation of the teeth and the complication of 

 their folds. In both of these characters the Grevy zebra 

 is slightly more advanced than any of the other living horse- 

 like ungulates. Its coloration is distinctive but nearer, 

 perhaps, that of the mountain zebra, E. zebra, which shows 



