578 MR. M. F. WOODWARD ON [May 5, 
addition the entire absence of any labial growth in connection 
with pm. 1, which one might naturally expect to find if the fune- 
tional pm. 1 was ppm. 1, andif Bate’s specimen was an exceptional 
one 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 ontogeny alone. That the 
time of appearance of the enamel-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 pe. ; 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 should 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 (81), who regarded the 
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 with 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 the more anterior molars. 
The only doubt arising in my mind as to whether 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 Erinaceus; 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, we 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. 
I 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 Hrinaceus 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 premolar in this genus as pm. 2 may be quite 
