24 THE PRJEVAL8KY HORSE. 



cahallus. Not a single skull proved an exception to Franck's rule, 

 although the difference between the distance from the vomer to the 

 foramen magnum, and the distance from the vomer to the palatine 

 bone, varied considerably (from 9 to 35 mm.) in different skulls. 

 In this respect all the specimens of E. prjevalskii, without excep- 

 tion, exhibit the same peculiarity as does the domestic horse ; but 

 in both young and old animals the variation in the difference of 

 the two diameters is less marked, being from 7 to 15 mm. 



All the results I have obtained from the examination of skulls 

 of the ass allow me to confirm Franck's conclusions. The distance 

 from the foramen magnum to the vomer was less, in all the skulls 

 examined, than that from the vomer to the palatine bone ; the 

 dift'erence ranging from 6 to 15 mm. In the single skull of the 

 mule which was at my disposal, Franck's index resembled that of 

 the horse. The distance from the foramen magnum to the vomer 

 was about 5 mm. greater than the distance from the vomer to the 

 palatine bone. In the onager and kiang, Franck's index was of 

 great interest, because of its extreme variability. In some speci- 

 mens it had the characters of that of the horse's skull ; in others 

 it resembled that of the ass. Of seventeen skulls of the onager, 

 four showed the horse type of index ; and the remaining thirteen 

 the ass type. Of eleven skulls of the kiang, six had an index 

 resembling that of the horse ; and the remaining five, the index of 

 the ass. With respect to Franck's index in E. • hemionus, 

 Nehring* remarks that the Dshiggetai (probably a Thibet kiang) 

 is closely allied to the domestic ass. In Nehring's tables, E. 

 hemionus is represented by only one skull, which, as it happens, 

 shows the characters of the ass and not of the horse. Measure- 

 ments of many skulls show that Franck's index is not character- 

 istic for the Asiatic wild ass ; though, according to my own 

 measurements at any rate, it is one of the best features for the 

 distinction of the skull of the donkey from that of the horse. 



From the measurements of the skull of an African ass, Nehrinc; 

 obtained a Franck's index like that characteristic of the horse. 

 Endeavouring to explain this anomaly, he says that joerhaps it is 

 the consequence of prolonged domestication. I prefer, rather, to 

 descry in this case an indication that E. asinus is allied to an 

 ancestral form from which the horse and the Asiatic ass have 

 descended. In such a form, there would be an approximation, in 

 part to the horse type of Franck's index, in part to the ass type ; 

 such as has been observed in the Asiatic wild ass. Perhaps E. 

 hemionus is more closely allied to that original general type from 



* Op. cit. p. 111. 



