THE TEETH OF MAMMALS. 293 



and by the introduction of secondary inflections, patterns 

 apparently dissimilar are arrived at. 



In the molar tooth of the horse, arrived at by a modifi- 

 cation of the Palseotherium type, we have a surface con- 

 stantly kept rough by the varying hardness of its different 

 constituents. 



In a worn tooth, we have upon a general field of dentine 



FIG. 122 



two islands of cementum, bounded by tortuous lines of 

 enamel, and on the inner side a sort of promontory of 

 dentine, bounded by enamel. The tortuous lines of enamel 

 by virtue of their hardness will, at all stages of wear, be 

 more prominent than the dentine or the cementum, and 

 will hence maintain the efficiency of teeth as grinders. 



The -patterns of grinding surface thus produced, are very 

 constant for allied species, so that an individual tooth of a 

 herbivore may sometimes be correctly referred to its genus, 

 and always to its family. 



But as it will be necessary to recur to this subject from 

 time to time, it will suffice for the present to point out that 

 such correspondences do exist, and that all the complexities 

 of pattern found, may, in practice, be reduced to some few 

 types. 



(!) Molar tooth of Horse, showing the characteristic pattern of its 

 grinding surface. 



