THE PRJEVALSKY HORSE. 29 



cabal/us, and more nearly approximates the Asiatic ass, in which 

 the average Avidth of forehead amounted to 206 mm. The width 

 in the several skulls of E. hemAonus varied from 198 to 209 mm. 

 In the domestic horse, as Nehring has shown, the variation in the 

 width of the forehead is very great when measurements are made 

 of skulls taken from a number of different breeds. Nehring 

 measured forty skulls of E. caballus, in which, according to his 

 tables, the frontal width varied from 175 to 255 mm. Of the forty 

 skulls, the width of the forehead was over 230 mm. in six speci- 

 mens ; and the average for the whole number was 218 mm. This 

 clearly shows that the absolute figures of measurements of the 

 skull of the domestic horse are of little value when they are to 

 be compared with those obtained by measurement of the skulls 

 of wild species of Equidxr. In the constancy of its frontal width, 

 the Prjevalsky horse takes first jjlace among wild species. 



Tlie ratio of the frontal width to the basilar length, and to the 

 length of the vertex of the skull, indicated in Nehring's Indices I. 

 and II., and the position of the eyes (Index III.) are fairly stable 

 in E. prjevalskii. Index I. varies from 232.6 to 244.9 ; but the 

 latter figure refers to a skull which was remarkable in having an 

 exceedingly narrow forehead. In general, I can agree with Czerski, 

 who says that E. prjevahhii belongs to that order of horse in 

 which the forehead is of medium width ; or, in other words, where 

 Index I. varies from 226 to 240. In the collection in the Zoologi- 

 cal Museum is the skull of a ten-months' foal (Skin, No. 3,072), 

 which assuredly belongs to the type of horse with a broad forehead, 

 since it has an Index I. of 212.5. As I have only this single skull 

 of so young an animal, I cannot venture to assert that the 

 Prjevalsky horse, at an early age, has a broad forehead like the 

 ass, which becomes of medium width later on. I must point out, 

 however, that this skull differs in a remarkable manner from all 

 the other skulls of E. ■prjevalskii, in which the first frontal index 

 is of conspicuous constancy. In regard to the frontal index (Index 

 I.), the Prjevalsky horse occupies an isolated position among the 

 various species of Equus. The index is greater in E. prjevalskii 

 than it is in the Asiatic ass and in the African ass ; that is, the 

 forehead is of moderate width. In the asses, on the other hand, 

 the forehead is broad, except in some rare specimens — mostly 

 young animals — of the Asiatic ass where the frontal index exceeds 

 225. In one specimen, indeed, I found an index w^hich decidedly 

 approached that of the narrow-forehead type of skull (239 !). Those 

 Prjevalsky horses in which the index is fairly constant (234 on an 

 average) resemble, so far as the forehead is concerned, the breeds 

 of the domestic horse in which the forehead is of medium width. 



