32 THE PEJEVALSKY HOESE. 



more closely, since the relative length of the cranium was as 

 100 : 210-234. Therefore, although the Prjevalsky horse and the 

 domestic horse have the greatest absolute cranial length, neverthe- 

 less, in proportion to the total length of the skull, the cranium is 

 least developed in these animals. 



The height of the skull, as determined by the distance from the 

 lower margin of the foramen magnum to the occipital crest, was 

 greater in E . ijrjevahkii than in any other member of the genus, 

 and varies from 102 to 110 mm. In the Asiatic ass the maximum 

 height amounted to 97 mm. only, and consequently did not reach 

 the minimum of E. prjevalskii. In the donkey the height of the 

 cranium was even less ; its maximum being only 90 mm. In the 

 Somaliland ass alone did the height amount to 100 mm. Among 

 the representatives of E. cahaUus, only the Tarpan (No. 521) ex- 

 ceeded the Prjevalsky horse in the height of the cranium ; and in 

 this animal I obtained my maximum measurement, namely, 

 117 mm. 



The foregoing figures lead to the conclusion that the cranium 

 of E. yrjivahMi is very largely developed in all directions. If 

 its relative length (as compared with the total length of the skull) 

 is less than in other species, this is explained by the very con- 

 siderable length of the skull as a whole, which is exceeded only in 

 some domestic horses and in the zebra. 



The position of the orbital cavities, determined by Nehring's so- 

 called " ocular lines," and shown by his ocular index (Index III.), 

 does not always agree with Index IV. This might be expected a 

 'priori, since Index IV. refers to the cranium. In E. prjevalskii, 

 the ocular index varied within very narrow limits, namely, from 

 190 to 197 ; or, if one of the young skulls be included, from 190 

 to 199. In the Asiatic ass, on the contrary, the variation was 

 very marked, namely, from 183.5 to 201 ; and in the African ass 

 it was still greater — from 155 (in the foal. No. 519) to 182.4. In 

 the domestic horse the range was less — from 174.4 to 195.9. In 

 consequence of its great variability, the ocular index cannot possess 

 so great an importance in systematic classification as do Indices 

 I., II. and IV. On this account Czerski * has already pointed 

 out that the ocular index is much less important than Indices I. 

 and II., and has proposed to replace it by a facial index, as before 

 mentioned. 



The extent of the face was determined (1) by its length from 



the incisors to the posterior end of the inter-nasal suture, and by 



Index IV., which gives the proportion of the facial length to the 



length of the vertex of the skull as a whole ; (2) by its breadth 



* Op. cit. p. 378. 



