6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 6l 



skull. Yet one of the four skulls of Grevy's zebra has the same 

 relative breadth as one of Grant's zebra. When, however, the aver- 

 age of the four skulls of each species is taken, that of the skulls of 

 Grevy's zebra equals 38.6, that of the skulls of the other zebra 40.5. 

 It is, therefore, pretty certain that measurements made on a large 

 number of skulls of the two species will show, as regards breadth, a 

 decided specific difference. Nevertheless, individuals will be found 

 which trespass the limits. 



It will be noted that the relative breadth of the skull of the Przeval- 

 sky horse is much higher than that of either of the zebras, equaling 

 that of the Arabian horse here measured. The specimens of Equus 

 niobrarensis stand above all the others. On looking through the 

 figures representing the facio-cephalic index, it seems to be hopeless 

 by this means to distinguish species. The average for Equus grevyi 

 amounts to 76.5; that of Grant's zebra, 75.1; that of Przevalsky's 

 horse, 76.7. These figures seem, however, to exclude the possibility 

 that the Alaskan skull belonged to Equus prscvalskyi. 



One of the skulls of the Przevalsky horse has a very low cranio- 

 cephalic angle, and this reduces much the average for this species. 

 The skull was that of a horse much younger than the other two, and 

 this may have something to do with its shortness behind the orbits. 

 The average for Grevy's zebra is 35.5; for Grant's, 37.1 ; for the 

 Przevalsky horse, 35.3. All the four skulls of E. grevyi have this 

 index less than any of those of Grant's zebra. 



An examination of the figures representing the palato-cranial 

 angle shows that there is, in some cases, great variation among mem- 

 bers of the same species. Among the three individuals of Equus 

 przevalskyi the range of variation is not wide, although there is much 

 difference in age. The specimen numbered 32686 is regarded by 

 Mr. Chubb, of the American Museum of Natural History, who has 

 given much attention to the osteology of horses, as having had the 

 age of thirteen years; number 136, twelve years; and number 71, 

 four and one-half years. The differences do not, therefore, appear 

 to be due to age, the youngest horse standing between the other two. 

 In the case of the skulls of Equus grevyi there is a range of n°. 

 The youngest, No. 163227, yet retains the milk-teeth, but has the 

 second upper molar through the bone. It has the highest angle, 29 . 

 The next in age, No. 163238, had the last molar just beginning to 

 wear and the third incisors not yet cut; its angle is 21 . The other 

 two animals were of practically the same age, with all the permanent 

 teeth in use and w T ith the incisors yet cupped. The angles are, as 

 seen, respectively, 18 and 22.5°. 



