302 A MANUAL OF DENTAL ANATOMY. 



diphyodonts correspond to the single set of nionophyodonts, 

 so that the milk dentition, when it exists at all, is some- 

 thing superadded. 



Whether this be so is a question difficult to determine ; 

 from the facts advanced by Professor Flower, while they 

 stood alone, most people would, with little hesitation, concur 

 with his conclusions ; but the history of the development of 

 the teeth interposes a fresh difficulty. 



The tooth germ of the milk tooth is first formed, and the 

 tooth germ of the permanent tooth is derived from a portion 

 (the neck of the enamel germ) of the formative organ of the 

 milk tooth (see Fig. 67). Again, in most of those animals 

 in which there is an endless succession of teeth, such as the 

 snake, the newt, or the shark, each successive tooth germ is 

 derived from a similar part of its predecessor, the natural 

 inference from which would be that the permanent set, 

 being derived from the other, was the thing added in the 

 diphyodonts. 



The question cannot be finally settled until we know more 

 of the development of the teeth of the monophyodont cetacea : 

 thus it might turn out that in them also there are abortive 

 germs of milk teeth formed, which do not go on so far as 

 calcification, but which do bud off, as it were, germs or per- 

 manent teeth ; if such should prove to be the case, this 

 would bring their teeth into close correspondence with those 

 of the elephant seal. 



The investigation of these questions is further complicated 

 by the fact that there are quite numerous instances of " per- 

 manent " teeth, that is teeth unquestionably belonging to 

 the second set, which are shed off early, and do not remain 

 in place through the lifetime of the animal ; an example of 

 this is to be found in the Wart Hog (Phacochserus), which 

 loses successively all its premolars and the first and second 

 true molars, the last true molar alone being truly persistent. 



Sometimes nothing but a careful comparison of the teeth 



