630 



sets of teeth 18 ), so that the result arrived at was the only legitimate 

 one which could be drawn from the facts before me. 



Now with the possibilities of traces of 3 or 4 distinct sets of 

 teeth succeeding one another, a wonderful variety of theories may be 

 put forward. 



First the labial growth seen connected with the molar in Phoca 

 and Macropus may represent the pre-milk dentition of Leche, while 

 the molar may be interpreted as belonging to the true milk dentition, 

 the lingual growth of the dental lamina representing the rudiments of 

 the ordinary successional (second and third) set of teeth. This view 

 with regard to the last two structures is held by the majority of 

 recent writers. 



On the other hand Leche's exceedingly vestigial pre-milk denti- 

 tion may have completely vanished or what is quite possible may be 

 differently interpreted and there may be no pre-milk dentition, then 

 the labial growth must be considered to represent a vanishiug milk 

 dentition, the adult molars finding their homologes amongst the true 

 successional teeth (second or third set), while the lingual growth of 

 the dental lamina will represent the rudiments of a still later set of 

 teeth which never become functional in the Mammalia, i. e. Küken- 

 thal's and Leche's fourth set of teeth. 



The presence of vestigies of only one set of teeth in addition to 

 the functional molars in the Rodents, makes it exceedingly difficult 

 to offer any satisfactory explanation as to the homology of those 

 teeth in that group, especially so when we remember that it 

 is by no means conclusively settled as to which set the molar 

 teeth of any Mammal are to be refered. Nevertheless I am of 

 the opinion, contrary to that which most other authors have ar- 

 rived at, that owing to the peculiar conditions under which the 

 molar teeth are developed in most mammals, viz: — the extreme 

 shortness of the jaw of the foetus causing the relative large molar 

 germs to be displaced so as to loose all connection with the gum and 

 to become burried up in the angle of the jaw, the earlier sets of 

 teeth viz : — the doubtful pre-milk dentition and even the milk den- 

 titiou itself would stand a great chance of being aborted, reduced and 

 lost, while the later developing teeth viz: — those developed from 

 the deeper seated portion of the dental lamina would stand a better 



19) Kose had suggested the possibility of the presence of a third 

 set of teeth from certain abnormal cases in the human subject of the 

 presence of additional replacing teeth. But he had not then worked it 

 out derelopmentally, see Rose, Archiv f. mikr. Anat., Bd. XXXVIII, 

 p. 460, 1891. 



