FOSSIL HORSE. 



389 



Section of diseased lower molar. Equus 



Fig. 148. 



inner surface (fig. 147, c). The parietes of this cavity, 



composed of dentine and enamel of the natural structure, 



were from one to two lines p^ 14 ^ 



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 



pOrOUS texture of its Walls, fossilis. 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- 

 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. 1 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, overlying the chalk at Hessle, 

 near Hull, the mark (m) still remains, 

 showing that the tooth had belonged to a Horse not aged. 



Incisor of fossil Horse, 

 Drift, Hessle. Nat. size. 



