34 THE PEJEVAL8KY HOESE. 



much less extent in E. prjevalskii — it is not possible to draw con- 

 clusions regiirding peculiarities of the skull of different species 

 from the height of the face. 



As Poliakof * has already demonstrated, the lower jaw of E. 

 prjetmlskii is veiy different in shape from that of the donkey and 

 the Asiatic ass, and resembles much more closely the mandible 

 of the domestic horse. Its inferior border is smooth and straight 

 (Fig. 8) ; whereas, in the donkey, it is curved and furnished with 

 prominences (Fig. 9). The fine illustrations contained in Poliakof's 

 monograph reveal, in the clearest manner, this difference between 

 the mandible of the ass and that of the horse. I can fully confirm 

 Poliakof's statements, and regard the peculiarity as being a very 

 constant feature by which the skull of the donkey may be dis- 

 tinguished from the skull of the horse. I can only add that, as 

 shown in Poliakof's drawing, between the horizontal and verti- 

 cal rami of the mandible of the donkey there is a very characteristic 

 indentation ; and that the inferior border of the jaw is much 

 thicker in the ass than in the horse. These features are so con- 

 stant that, with their aid, one can, with a fair amount of certainty, 

 decide whether a given skull belongs to a horse or to a donkey. 



The dimensions of the lower jaw, the length of the diastema 

 between the incisor and cheek teeth, and the len^h of the series 

 of cheek teeth are not all valuable as distinguishing characters. 

 The mandible of E. prjevalskii measured from 421 to 436 mm. 

 in length, and was, therefore, longer than the corresponding bone 

 in the other species. A similar length occurred only very excep- 

 tionally in the Asiatic wild ass (for example in No. 217, the only 

 skull of E. hemionus in which the lower jaw reached a length of 

 435 mm.). In none of the domestic horses measured by me was 

 the length of the jaw so great as this (the maximum was 434 

 mm.) ; although in heavy draught horses (Brabant breed), measured 

 by Nehring, the bone attained a length of 515 mm. in a skull 

 with the enormous length of vertex of 628 mm. Among the wild 

 species of the genus, E. prjevalskii is distinguished by the 

 possession of the longest lower jaw. 



Although, in individual cases, the height of the lower jaw showed 

 considerable variation, it was found to be a distinguishing feature 

 of the skull of the Prjevalsky horse. In full-grown animals, it 

 varied from 206 to 234 mm. So great a height was not found in 

 any skull of the Asiatic ass (maximum 215 mm.), nor in the 

 African ass (the lower jaw of the Somaliland ass alone reached a 

 height of 214 mm.). Only in the skull of the domestic horse, and 

 only in a few of those measured,' did the height of the jaw exceed 



* Op. cit. p. 12. 



