264 COMPARATIVE ZOOLOGY OF CHORDATES 



Another difference between the teeth of mammals and 

 those of other vertebrates lies in the fact that they arise in two 

 sets, or, in other words, they are replaced once only (except for 



A 



- 4.-:""^ 



<T 



Fig. 123. 



-Diagrams showing the^relation of the mammalian to other 

 modes of tooth-succession. (After Bolk.) 



A, diagrammatic view of the outer side of the dental lamina (dl) of the 

 lower jaw, showing the alternation between tooth-germs at the side (s) and 

 at the base (b) of the dental lamina. B, diagrammatic representation of 

 tooth-replacement in reptiles ; the teeth formed from the tooth-germs at 

 the side of the dental lamina are shaded : those formed from the tooth- 

 germs at the base of the dental lamina are white ; the tooth-germs produce 

 several teeth, which replace other teeth formed originally from the same 

 tooth-germ as themselves. C, diagrammatic representation of tooth- 

 replacement in mammals ; each tooth-germ produces one tooth only, and 

 the teeth formed from the tooth-germs at the base of the dental lamina 

 replace those formed from the tooth germs at the side of the dental lamina. 

 e, ectoderm. 



the molars which are not replaced at all). Other vertebrates 

 have perpetual replacement of teeth as and when the exist- 

 ing ones wear out. The mammalian condition is called 



