THE PEJEVALSKY HOESE. 39 



their form and the arrangement of the tissues of their wearing- 

 surfaces. For this reason, no constant specific characters can be 

 cited by which the teeth of different species can be distinguished. 

 In the incisor teeth, the degree of narrowing from the wearing 

 surface towards the base of the tooth has been used as a character 

 by which the oriental may be distinguished from the occidental 

 horse (Wilckens) ; but Czerski has demonstrated its unreliability. 

 According to Franck, the folding of the enamel of the cheek-teeth 

 serves to distinguish not only the above races of the horse, but 

 also tlie ass (Avith straighter enamel folds) from the horse (in 

 which the folds are more complicated). But this also, according 

 to Nehring, is not constant. Concerning the relation of the length 

 of the cheek-teeth to their breadth as a further distinction between 

 the horse and the ass, I shall speak later. 



Referring to the teetli of fossil horses, Nehring says truly that 

 a comparison of the enamel folding can only be of value when the 

 cheek-teeth are in a state of moderate wear. I have confined my 

 attention to such teeth. 



The incisors of E. jirjevahliii (I consider adult animals only) 

 are distinguished from those of other horses by their considerable 

 size, as the following table shows : 



Table of Measurements of the Incisors of Equus 



Prjevalskii. 



Lower Teeth. 



No. mm. mm. mm 



.oiod: 33 ... 17 15 



^'^^^■|l3 22 ... 21 17 



5,216 j }' 



r oi . 1 l! 35 ... 18 16 



'^'-^^^ I 1,. 27 ... 18 16 



),213|}' 



12 

 14 



These dimensions show no essential differences from those 

 obtained by measurement of the teeth of the domestic horse and 

 the onager. 



The length of the diastema between the incisor and cheek-teeth 

 varied in the Prjevalsky horse from 77 to 90 mm. Although the 

 diastema of one specimen of onager now before me measures 95 

 mm., it is generally much shorter in this animal ; and it can be 

 stated, on the basis of averages, that the diastema in E. prjevaJsl-ii, 

 on account of its marked length, comes nearest to that of the 

 domestic horse, in which it reached 108 and 109 mm. in specimens 

 I have examined. 



