34 MAMMALIAN DENTITION 



tures upon the walls of the pharynx and the branchial arches 

 beyond the limit to which Embryology has taught us the exter- 

 nal skin has penetrated. It is plain then that teeth cannot in 



Fi o- 3 -Jaws of Shark to show formation of simple triangular teeth from dermal scales, 

 i i . J«» (After Dewey-Thompson.) 



every case be dermal in origin but may sometimes develop from 

 the surface layer of the alimentary canal itself. Thus we see 

 that structures finally similar may in the beginning arise in 

 quite different manner from dissimilar tissues. 



