TEETH AND THEIR VARIATIONS. 199 



a very imperfect condition. True teeth are found only 

 among the Vertebrate animals ; and we propose to take 

 a short glance at some of the most striking variations 

 displayed by them among certain groups of the class of 

 Mammals, although some mention will also be made of 

 those of the lower types. 



Curious as it may seern at first sight, the researches 

 of modern naturalists have now conclusively proved 

 that primarily teeth are neither more nor less than 

 special modifications of certain bony structures found 

 in the external skin of some of the lower Vertebrates, 

 of which the best known example is afforded by the 

 hard granules in the skin of the Sharks, commonly 

 known as shagreen. The teeth of some fishes depart, 

 indeed, but very slightly from these simple bony 

 granules ; and a gradual transition can be traced from 

 these simple teeth to the most complex types of teeth 

 found in the whole Vertebrate group. 



In speaking of these simple teeth as bony structures, 

 we take leave to employ the word bony in a somewhat 

 popular sense, since, strictly speaking, teeth are not 

 composed of true bone. Indeed, in the majority of 

 Vertebrates, teeth consist either of two or three per- 

 fectly distinct structural elements, which can be easily 

 recognised by the naked eye. Thus in the human teeth, 

 for instance, the outer surface of the exposed portion, 

 or crown, as it is technically called, is seen to be 

 uniformly coated with a bluish-white and highly 

 polished substance of exceeding hardness, conveniently 

 known as the enamel. On looking at the grinding 



