THE PRJEYALSKY HORSE. 25 



which sprang- the two species of horse (E. caballiis and E. lyrjeval- 

 shii) and the donkey (E. asinus). 



Unfortunately I have only a few skulls of the zebra at my dis- 

 posal (two of E. zebra ; one of E. grevyi ; one of E. biirchelli, and 

 one of E. hurchelli var. chapinani). So far as one can judge from 

 the measurements of so few specimens, Franck's index varies 

 greatly in the zebra. The index resembled that of the horse in 

 both specimens of E. zebra, and that of the ass in all the other 

 zebras.* Nehring's measurements,! which include those of 

 several specimens of E. zebra, indicate that Franck's index is not 

 always of the same type in this species ; since tw^o out of three 

 skulls exhibited the ass type of index, while, in the third, the index 

 resembled that of the horse. It is evident that, for neither the 

 zebra nor the Asiatic ass, is Franck's index characteristic or 

 diagnostic. 



The fronto-nasal region, consisting of the frontal, nasal and 

 lachrymal bones, forms a characteristic part of the skull, and 

 serves well as a feature by which a horse's skull may be distin- 

 guished from that of a donkey (but not from that of the Asiatic 

 ass). In this region, the nasal bones are of great importance, 

 since upon their configuration depends the shape of the frontal 

 and lachrymal bones, as well as the form of the naso-lachrymal 

 suture. Consequently, measurements of the nasal bones have been 

 included in the accompanying tables. Before reviewing these, 

 however, it will be well to discuss the individual bones and sutures 

 of the region. 



By the form of the naso-lachrymal suture, the skull of the horse 

 may be distinguished from that of the ass (Czerski).J In the 

 horse the suture is straight and nearly parallel with the long axis 

 of the skull ; but in the ass it is curved and convex towards the 

 lachrymal bone. Therefore, the inner border of the lachrymal 

 bone is straight in the horse, and concave in the ass. This pecu- 

 liarity is very constant, although in certain asses the degree of 

 the curvative varies somewhat ; and, in certain horses, the com- 

 monly straight suture may be curved. The unusual curve may be 

 directed either towards the lachrymal bone or towards the nasal. 

 Naturally, in the latter case, the border of the lachrymal l)one is 



* My measurement of the skull of the zelira No. Ill agrees with that given 

 by Poliakof. Czerski avers that Poliakof gave the figures in inverse order: that 

 is, that the distance from the foramen magnum to the vomer as compared -with 

 the distance from the vomer to the palatine bone should read as 103 : 111 ; and 

 not, as Poliakof states, as 111 : 103. I can, however, affirm that Poliakof is 

 correct, and that Czerski is in error. 



t Op. cit. pp. 98-101 and 111. 



X Op. cit. p, .399. 



