396 



UNGULATA 



conspicuous from its dark colour, and constitutes the " mark " by 

 which the age of the horse is judged, as in consequence of its 

 extending only to a certain depth, it becomes obliterated as the 

 crown wears away, when the tooth assumes the character of an 

 ordinary incisor, consisting only of a core of dentine surrounded 



by the external enamel 

 layer. It is not quite so 

 deep in the lower as in 

 the upper teeth. The 

 canines are either quite 

 rudimentary or entirely 

 absent in the female. In 

 the male they are com- 

 pressed, pointed, and 

 smaller than the incisors, 



/ / Id e * from Avhich they are 



separated by a slight in- 

 terval. The teeth of the 

 cheek series are all in 

 contact with each other, 

 but separated from the 

 canines by a considerable 

 toothless space. The 

 anterior premolars are 



PIG. 165. Longitudinal and transverse section of upper quite rudimentary, often, 



incisor of Horse, p, Pulp cavity ; d, dentine or ivory ; e, especially in the lower 



enamel ; c, outer layer of cement ; cf, inner layer of cement, , j -, , , 



lining a, the pit or cavity of the crown of the tooth. ] aw > not developed at all, 



and generally fall by the 



time the animal attains maturity, so that there are but six func- 

 tional grinding teeth three that have predecessors in the milk- 

 dentition, and hence are considered as premolars, and three true 

 molars, but otherwise, except the first and last of the series, 

 not distinguishable in form or structure. These teeth in both 

 upper and lower jaws are extremely long-crowned or hypsodont 

 (Fig. 158), successive portions being pushed out as the sur- 

 face wears away; a process which continues until the animal 

 becomes advanced in age. The enamelled surface is infolded in a 

 complex manner (a modification of that found in other Perissodac- 

 tyles, see Figs. 155, 167), the folds extending quite to the base of 

 the crown, and the interstices being filled and the surface covered 

 with a considerable mass of cement, which binds together and 

 strengthens the whole tooth. As the teeth wear, the folded enamel, 

 being harder than the other constituents the dentine and cement 

 forms projecting ridges on the surface arranged in a definite 

 pattern, which give it great efficiency as a grinding instrument (see 

 Fig. 157, b and c). The free surfaces of the upper teeth are 



