SOME INTERNAL STRUCTURES 75 



to negative this theory, for in that animal an indefinite 

 succession of complicated teeth occurs.* 



In almost all Mammalia the individual is provided 

 with two sets of teeth ; there is the dentition found 

 in the young ; this is later replaced by the dentition 

 of the adult. The two sets of teeth are spoken of 

 respectively as the "milk" and the "permanent" 

 dentition. This is characteristic of the mammalia, 

 and distinguishes them from lower vertebrates where 



O 



there is not this merely double dentition ; new teeth 

 in the lower vertebrates are formed as they are 

 wanted. If a mammal loses one of the teeth of 

 the second series that tooth is not replaced. The 

 relative importance of these two sets of teeth varies 

 much. The milk teeth are sometimes only developed 

 as rudiments, never of functional use, while in other 

 cases the milk teeth persist for a long time, and are 

 very distinctly functional. It has been even attempted 

 to be shown that in the Marsupials it is the permanent 

 dentition which is suppressed and only represented by 

 rudiments, while the teeth of the full-grown animal 



o 



are the persistent milk teeth. This general character 

 of the Mammalia has been described as "Diphyodont," 

 and it was thought that by this the majority of 

 mammals were to be distinguished from some that 

 have but one set of teeth, and were accordingly to 

 be termed Monophyodont. In some of the Edentata 

 (the Sloth) it is still believed that only one set of teeth 

 is ever produced ; and the same view was originally 

 held about the toothed whales. There is, however, 

 * See LYDEKKER and THOMAS, Proc. Zool. Soc., 1897, p. 595. 



