THE TEETH OF MAMMALS. 297 



found nearest to the articulation, where the greatest force 

 is exerted. 



Thus " bunodont " animals, i.e. those that have rounded 

 conical cusps upon their short-rooted teeth, have a cylindrical 

 condyle ; selenodonts, or those with crescentic ridges on the 

 molars, have a condyle which is expanded and plane, while 

 lophodonts, or those with transversely ridged teeth, have a 

 globular condyle. 



This correspondence pointed out between the condyle, the 

 movements of the jaw, and the form of the teeth does exist, 

 but it is less easy to see how it is brought about. The 

 simple mechanical explanation that the teeth are drawn out 

 into these forms, hardly conveys much information, seeing 

 that the tooth before it is subjected to these influences, is 

 quite finished, and its form, such as it is, is unalterable : 

 while to effect an alteration in the form of a masticating 

 surface an influence must be brought to bear upon the 

 tooth germs at an exceedingly early period. It might with 

 equal justice be said that the crown of the tooth being- 

 formed thus had influenced the excursions of the jaw, and 

 so modified the condyle. 



THE MILK DENTITION. 



Some thirty years ago Professor Owen called attention to 

 the fact that those mammals in whom thd teeth situated in 

 different parts of the mouth were alike in form (homodonts), 

 developed only one set of teeth, and to indicate this charac- 

 teristic he proposed for them the term " monophyodonts." 

 Those, which, on the contrary, had teeth of different size 

 and form in various parts of the mouth (heterodonts), de- 

 veloped two sets of teeth ; a " milk " set, which was dis- 

 placed by a permanent set, and this peculiarity he expressed 

 by the term " diphyodonts." As originally set forth, the 

 terms homodont and monophyodont were interchangeable, 



