HIGH SCHOOIi ZOOLOGY. 173 



reappearing in the permanent dentition, A. peculiarity of 

 the incisors is that the surface enamel is folded in like the in- 

 verted finger of a glove, the I'esult being a ring of enamel which 

 constitutes the mark of the incisors, until, in the aged horse, the 

 tooth has been worn down below the fold. In all the members 

 of the family, the hair of the mane and tail is long, and there 

 are present callosities in the skin near the knees and hocks, but 

 in the asses and zebras, the hair is only long at the tip of 

 the tail, and the callosities are only present on the fore-legs- 

 The zebras are South African forms distinguished by black 

 stripes on a cream-coloured ground ; the asses occur in North 

 Africa, "Western and Central Asia, the North African species 

 being probably the source of the domestic ass. Although 

 horses are found in a feral condition (i.e., appai-ently wild, but 

 really only secondarily so) in Asia and South America, it is un- 

 certain whether the original stock still exists in a wild condi- 

 tion ; some recent investigations, however, in the high table-lands 

 of Thibet point to the conclusion that such is the case. The 

 South American horses were imported by Europeans, but it is 

 not to be supposed that the New Woild was until the time of 

 its discovery uninhabited by horses. Fossil remains of true 

 horses show that they were abundant in America long before 

 their importation from Europe, and from the various Tertiary 

 strata numerous ^epresentati^•es of the family have been dis- 

 covered, so that the American fossil Equidse are much more 

 numerous than the European forms. Those from the lower 

 Tertiaries had both a more complete dentition and also a greater 

 number of toes, recalling in this respect the genus Coryphodon 

 (§ 25) ; but as we study the forms which occur in the higher 

 Tertiary strata we find a gradual loss first of the fifth, then of 

 the second and fourth toes until the third alone is left 

 with the rudiments referred to above. The earlier Equidse 



wei'e small, about the size of a fox, but they gained in size 

 t,ro 



