SKELETON AND TEETH 97 



would be of no advantage. In several instances, always as 

 a secondary specialization, the enamel does not cover the whole 

 crown, but is arranged in vertical bands, it may be on one 

 side only, or at intervals around the tooth. The scalpriform 

 incisors of the rodents, already alluded to, have the enamel 

 band on the front face of the tooth ; the softer dentine behind 

 wears away more rapidly, keep- 

 ing the cutting surface bevelled, 

 like the edge of a chisel, while 

 the hard enamel forms the sharp 

 edge. In some instances the en- 

 amel is absent altogether and the 

 teeth are composed entirely of 

 dentine, as in the elephant tusk. Fig 47. -Section through a lower 



molar of the Indian Elephant (Ele- 

 In all the Edentata, such as phas maximus). Enamel, heavy 



sloths and armadillos, both liv- black ; dentine, white; cement, 



' horizontal lines. 



ing and extinct, that have any 



teeth at all, the teeth have no enamel, but in some of the 

 fossil forms the place of the missing enamel is taken by a 

 harder dentine and thus the effect of differential hardness 

 is secured. 



(3) The cernent is simply bone, both chemically and in 

 microscopic structure ; it is not quite so hard as dentine, but 

 it is less affected by the fluids of the mouth and the juices of 

 the food. In the brachyodont or low-crowned tooth, such as 

 a human molar, the cement merely forms a sheath over the 

 roots and does not appear upon the crown, but in many 

 hypsodont teeth, those of horses and elephants, for example, 

 the cement completely encases the entire tooth in a thick layer, 

 filling up all the depressions and irregularities of the enamel 

 surface and making a freshly erupted and unworn tooth look 

 like a shapeless lump. When the cement and the enamel 

 covering are partially worn through, the masticating surface 

 is made up of three distinct substances, each having a dif- 

 ferent degree of hardness and thus, through unequal wear, 



