244 STRUCTURE AND ADAPTATION OF TEETH 



tooth, in which the vertical layers of dentine, 

 enamel, and cementum, are intermi:xed. 



The various patterns in which the folds are 

 arranged, serve to distinguish the several genera 

 of animals in which these compound teeth occur. 

 We see a beautiful example of design in this 

 arrangement. 



It is evident that this interfolding of parts 

 differing so much in hardness, must produce a 

 constantly roughened surface, which is maintained 

 by the unequal wearing down of the elements of 

 the tooth. The cementum is worn into deep 

 depressions, while the dentine is more moderately 

 wasted, and supports the ridges of enamel, which 

 always retain their sharpness. The folds are 

 arranged in the ruminants in two double cres- 

 cents, the convexity of which is turned inwards in 

 the upper, and outwards in the lower jaw : the 

 object of which is to enable them reciprocally to 

 act on each other in the most effective manner. 



A peculiar kind of stone is selected for the con- 

 struction of millstones, and is valuable on account 

 of the harder particles disseminated throughout 

 its softer constituents. The miller is obliged fre- 

 quently to dismantle and pick or dress the cutting 

 surface of the stone, to ensure its efficiency ; but 

 in the teeth of the gramnivorous animals, we see 

 the equivalent of a self-dressing millstone. 



The solitary row of front teeth is more perfectly 

 adapted to its purpose than would at first appear. 



