578 Mil. M. F. WOODWAED ON [^'^"■V 5, 



addition the entire absence of any labial growth in connection 

 with pm. 1, which one might naturally expect to find if the func- 

 tional pm. 1 was ppm. 1, and if Bate's specimen was an exceptional 

 ono in which dpm. 1 had been retained. 



One of the greatest difficulties met with in the study of tooth 

 ontogeny is the want of a sure method for the determination to 

 which set a given tooth belongs, for we may be dealing with a 

 retarded member of an early set or an accelerated development of 

 a later series, and, so far as I can judge, the identification can only 

 be made through a study of the comparative morphology and 

 phylogeny of the tooth, and not by its outogeny alone. That the 

 time of appearance of the euamel-organ does not help us is well 

 seen in the Mole, where the germ of pm. 1 appears after the other 

 milk-teeth and at the same time as pc. ; but this latter tooth appears 

 long before the other permanent teeth, so that if we took the time 

 of appearance of these tooth-germs as a criterion we sliould have 

 to conclude that the deciduous incisors, canines, and three posterior 

 premolars belonged to one set, the 1st premolar and permanent 

 canine to a second set, and the other permanent teeth to a third 

 series, a conclusion which, I think, condemns itself in the mind of 

 all those who have studied this subject. Such a suggestion was 

 put forward many years ago by Wortman (31), who regarded tlie 

 four molars of the Placentalia as belonging to four distinct sets 

 of teeth ; this view does not appear to have met with any general 

 recognition, it being more natural to suppose that the dental 

 lamina though temporarily fused wilh the germs of the anterior 

 molars yet retains its individuality and grows back with the elon- 

 gation of the jaw to form fresh teeth belonging to the same series 

 as tlie more anterior molars. 



The only doubt arising in my mind as to vvhetlier I am right in 

 referring the first premolar, in the Mole and in all animals where it 

 is only known in one dentition, to the milk-series and so terming 

 it dpm. 1, is due to the appearance seen in Erinacexis ; for if in 

 that genus the apparent tooth-vestige which I have mentioned {ante, 

 p 562) as occurring between the two posterior upper premolars really 

 represents a lost premolar, then the anterior premolar of Erinaceus 

 is the true pm. 1 ; and as further I have shown that the deciduous 

 predecessor of that tooth is a vestigial structure, the functional 

 tooth must be referred to the replacing dentition. Consequently, 

 if the above premises be true, wo have here an example of the 

 suppression of dpm. 1 and a persistence of ppm. 1, a conclusion 

 antagonistic to that which I have arrived at concerning this tooth 

 in the Mole, and I could only suppose that the homology of this 

 tooth (pm. 1) varies in different and closely related animals. 



1 have thought it only fair to give this possible objection to my 

 view here, but, as I have already mentioned, this supposed vestige 

 of pm. 3 in Erinaceus is very slight and has not been observed by 

 Leche in any of his stages ; so it is highly probable that this struc- 

 ture has no morphological importance, and Leche's identification 

 of the 1st functional preiriolar in this genus as pm. 2 may be quite 



