446 SCIENCE PROGRESS. 



are traces of a third set of teeth succeeding the replacing 

 set, so that together with the premilk series we appear to 

 have in mammals traces of four dentitions, viz. : — - 



1st, or premilk dentition — minute calcified teeth never 

 functional, present in Myrmecobius) 



2nd, or milk dentition — generally functional, the per- 

 manent dentition of marsupials and cetacea, more 

 or less temporary in the higher mammals. 



3rd, or replacing dentition — functional in most mammals, 

 rudimentary in marsupials and cetacea. 



4th dentition (?) — rudimentary in Phoca (?), occasionally 

 functional in man (?). 



While Leche maintains the view that the molars belong 

 to the milk dentition and regards them as simple teeth, 

 both Kukenthal and Rose consider that they are formed 

 by the fusion of a number of distinct conical reptilian teeth. 

 This idea is not a new one, it having been suggested by 

 Gervais (36), Gaudry (37) and Dybowski (38) and others to 

 explain the structure of the elephant's and ungulate's molars, 

 but Kukenthal was undoubtedly the first to apply it to all 

 mammals. Rose has taken up this view very strongly, and 

 he attempts to prove it by embryological evidence, especi- 

 ally by what he has observed in the chameleon (39), where 

 the back teeth are each composed of three cones, which, 

 according to him, arise independently of one another, this 

 is true to a certain extent ; but these cusps develop under 

 a common enamel organ and there is no indication of 

 their ever having possessed independent organs as would 

 have been the case if they were distinct teeth, there being 

 merely a differentiation of the cylindrical enamel epithelium 

 over each cusp, which Rose considers sufficient evidence 

 in favour of his view. Kukenthal professes (35) to have 

 seen in the walrus the fusion of two rudimentary molar 

 germs into one, but unfortunately he does not figure this 

 interesting find. He, however, puts forward this view of 

 the molar tooth genesis as a theory drawn particularly from 



1 This premilk dentition is probably analogous to that seen by Leche 

 in Iguana (40) and by Rose in the crocodile (41;. 



