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 Equidcz , 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 
