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, 
from which 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 
Fic. 165.—Longitudinal and transverse section of upper Quite rudimentary, often, 
incisor of Horse. p, Pulp cavity ; 4, dentine or ivory ; e, especially in the lower 
enamel ; c, outer layer of cement; c’, inner layer of cement, - 
lining a, the pit or cavity of the crown of the tooth. Jaw, 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 ina 
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 
