THE PKJEVALSKY H0E8E. 23 



difYerence between the skull of the horse and that of the ass, and 

 give interesting results when applied to the skull of the Asiatic 

 wild ass. The ratio which the two diameters bear to each other 

 will be indicated in the following by FrancKs index. 



The most important point in the study of the skull of E. 

 prjevalshii (Fig. 6) is its comparison with the skulk of other 

 species of the genus Equus, namely, the horse, the Asiatic ass and 

 the African ass. In E . jjrjevahkii we liaA'e to do with a form 

 which, in many respects, is intermediate between the horse, on the 

 one hand, and the avss (Fig. 7) (including the Asiatic wild ass), on 

 the other. On this account, as ace rate an examination as possible 

 must be made of each feature which can help to solve the question 

 of relationship. Franck * alludes to nine characteristics which, 

 in his view, represent points of difference between the horse and 

 the ass. Because all of them cannot be shown to be suitable for 

 determining the classification of the different species of the genus 

 Equii-^, I shall only mention one of Franck's characteristics — to 

 which reference has alreadj^ been made — namely, the ratio the 

 distance from the foramen magnum to the vomer bears to the 

 distance from the vomer to the posterior border of the palatine 

 bone. Czerski f who was also convinced of the inadequacy of 

 the majority of Franck's characteristics, gave such of them as by 

 careful test lie had proved to he most important. These referred 

 to the shape of the nasal and lachrymal bones, and the form of the 

 naso-lachrymal suture. In the ass, the superior (nasal) border of 

 the lachrymal bone is concave, whereas in the horse it is straight, 

 A further difference is the position of the infra-orbital foramen, 

 which is much farther from the nearest point on the na.so-maxillary 

 suture in the ass than in the horse. In the horse the post-orbital 

 process of the frontal bone is three-sided : in the ass it is oval 

 and compressed. 



There ai'e, therefore, a series of craniological features which 

 serve to differentiate the skulls of the two species of the genus 

 Equus. These are : Franck's index, the form of the nasal and 

 lachrymal bones and of the suture between them, and the shape 

 of the post-orbital process of the frontal bone. To these may be 

 added less important features — such as the shape of the lower 

 jaw, the length of the series of the cheek-teeth — to which reference 

 will be made later. 



Franck's index, and its significance, can be verified by measure- 

 ment. The correctness of Franck's conclusion was confirmed by 

 the examination of thirteen skulls of different breeds of E. 



* Op. cit. p. 41. 



t Op. fit. p. 399-402. 



