FOSSIL HORSE. 389 
inner surface (fig. 147, ¢). The parietes of this cavity, 
composed of dentine and enamel of the natural structure, 
were from one to two lines Fig. 147. 
and a half thick, and were 
entire and imperforate. 
The water percolating the 
stratum in which this tooth 
had lain, had found access 
to the Cavity through the Section of diseased lower molar. Lquus 
porous texture of its walls, — /esséis. Cromer. 
and had deposited on its interior a thin  ferruginous 
crust, but the cavity had evidently been the result of some 
inflammatory and ulcerative process in the original formative 
pulp of the tooth, very analogous to the disease called 
‘spina ventosa’ in bone. The incisors of the Horse are 
distinguishable from those of the Rumi- Fig. 148. 
nants by their greater curvature, and 
from those of all other animals by the 
fold of enamel, which penetrates the 
body of the crown from its summit, 
like the inverted finger of a_ glove. 
When the tooth begins to be worn, 
the fold forms an island of enamel, 
inclosing a cavity partly filled by 
cement, and partly by the discoloured 
substances of the food, and is called 
the ‘mark.’ In aged Horses the incisors 
are worn down below the extent of the 
fold, and the ‘mark’ disappears. In 
the incisor tooth (fig. 148) from drift 
gravel, overlymg the chalk at Hessle, _ Ineisor of fossil Horse, 
Drift, Hessle. Nat. size. 
near Hull, the mark (sm) still remains, 
showing that the tooth had belonged to a Horse not aged. 
