356 



ANATOMY OF VERTEBRATES. 



of the third permanent incisor, or ( corner nipper,' the ( colt ' be- 

 comes a ' horse,' and the ' filly ' a ' mare,' in the language of the 

 horse-dealers; after the disappearance of the 'mark 'in the in- 

 cisors, at the eighth or ninth year, the horse becomes ( aged.' 



The modifications which the 

 upper molars of Hi/rax, fig. 

 286, present, as compared with 

 those of Paleotherium, will be 

 readily understood by the re- 

 3 marks in the section on the 

 homologies of the grinding sur- 

 face, as illustrated by figs. 268- 

 2 70. The present genus is a mi- 

 niature form of the family, and, 

 like the primitive eo- and mio- 



therium), retains large incisors, 

 with a type molar series, e.g. 



Incisive dentition, 5-year old Horse. 

 Lower jaw. 



. 1.1 o.o 4.4 



\m'- - = 32. 



Dentition, upper jaw (Hyrax). 



There are no canines. As to the incisors in Hyrax or Rhinoceros 

 the species vary, not only in regard to their form and proportions, 



286 but also their existence ; 



and in the varieties of 

 these teeth we may dis- 

 cern the same inverse 

 relation to the develop- 

 ment of the horns which 

 is manifested by the ca- 

 nines of the Ruminants. 

 Thus, the two-horned 

 Rhinoceroses of Africa, which are remarkable for the great length 

 of one (Rh. bicornis, Rh. simus) or both (Rh. Keitloa) of the 

 nasal weapons, have no incisors in their adult dentition; neither 

 had that great extinct two-horned species (Rh. tichorinus), the 

 prodigious development of whose horns is indicated by the singu- 

 lar modifications of the vomerine, nasal, and premaxillary bones, 

 in relation to the firm support of those weapons. The Sumatran. 

 bicorn Rhinoceros combines, with comparatively small horns, 

 moderately developed incisors in both jaws. The incisors are of 

 larger size in the unicorn Rhinoceroses (Rh. Indicus and Rh. Son- 

 daicus) ; still larger, relatively, in the hornless Acerotherium and 

 Hyrax, figs. 286, 287, i. 



