30 THE PRJEVALSKY HORSE. 



All that luis been said above retjarding the first frontal index 

 (Index I.) is also applicable to the second index (Index II.), which, 

 in the Prjevalsky horse, varied from 255.7 to 270. In this respect, 

 the eighteen to twenty months' old foals fully resembled the adult 

 animals which had already changed their teeth. The ten-months' 

 old foals possessed the smallest second frontal index. In the 

 Asiatic ass the index was smaller than in the Prjevalsky horse ; and 

 in the donkey it was still smaller, as might be expected from the 

 consideration of Index I. 



In the accompanying tables are given measurements of the 

 brain-region of the skull made in various directions. In the first 

 instance, the skull was measured along Nehring's ocular line ; and 

 thus the position of the orbital cavity, which is closely related 

 to the size of the cranium, was defined. Next, the width of the 

 cranium was measured at three different levels : (1) between the 

 superior margins of the two external auditory meati ; (2) above 

 the zygomatic processes of the temporal bones ; and (.3) behind 

 the orbital cavities. Then, the distance between the posterior end 

 of the inter-nasal suture and the occipital crest was ascertained, 

 and this distance referred to the length of the vertex of the entire 

 skull, taken as 100. What I shall call Index IV. was thus com- 

 puted. Finally, the height of the cranium was measured from the 

 lower margin of the foramen magnum to the occipital crest. In 

 this way the dimensions of the cranium were ascertained in all 

 directions. The relation which the posterior ocular line (distance 

 from the occipital crest to the most external point on the posterior 

 margin of the orbital cavity) bears to the anterior ocular line (dis- 

 tance from the most external point on the posterior margin of the 

 orbital cavity to the base of the middle incisors) is shown in the 

 so-called Ocular Index, or Index III. of Nehring. 



The width of the cranium of E. prjevalskii showed the following 

 variations. The diameter between the external auditory meati 

 varied from 104 to 115 mm. in young animals, and from 112 to 

 120 mm. in adults. In this diameter the skull of the Prjevalsky 

 horse exceeded not only the skull of the ass, in which (with the 

 exception of the Somaliland ass with a relatively enormous head) 

 the width varied from 100 to 109 mm., but also that of the Asiatic 

 ass, where it did not exceed 117 mm. In respect of the width of 

 the cranium between the auditory meati, as well as in other 

 features, B. prjevalskii most closely resembled E. cahaUus. 



Anteriorly, the skull of the Prjevalsky horse narrows somewhat, 

 but its width is always greater than in any other species of Equus. 

 In full-grown Prjevalsky horses, the width of the cranium on a 

 level with the zygomatic processes of the temporal bones, ranged 



