310 Mr. O. Thomas—Notes on Dr. W. Kiikenthal’s 
sible, to follow Dr. Kiikenthal in his homologization of the 
changing tooth of Marsupials with p.* instead of p.*, as it has 
usually been considered to be, even if the missing premolar 
has left no trace of its former presence in the position (next 
anterior to “p.°””) which I suggested it had most probably 
occupied. The problem as to the homologies with each 
other of the Placental and Marsupial teeth is one that will 
need much further, and especially paleeontological, evidence 
for its solution; but comparing the dentition of Triconodon 
with those of both groups, it is difficult to avoid coming to 
the conclusion (1) that the changing tooth of Marsupials is 
homologous with the changing tooth, the fourth premolar, of 
Triconodon ; (2) that the four premolars of Zritconodon are 
homologous with the four premolars of the typical Placental 
dentition *; and, as a consequence, (3) that the changing 
premolar of Marsupials is homologous with p.4 of Placental 
Mammals. 
But if once the primitive diphyodont theory be admitted, 
the homologization of the Marsupial molars with the milk 
series 1s as likely as with the permanent, for originally all 
the teeth would have been in duplicate, the posterior as well 
as the anterior, and either set would be as likely to be sup- 
pressed as the other. And furthermore, if this homology of 
Dr. Kiikenthal’s is confirmed, and it seems well founded, in 
all probability the same will prove true of the Placental 
molars T, which we have as yet no real reason for knowing 
to be serially homologous with the permanent more than the 
milk set. In fact any presumption there may be one way or 
the other is rather in favour of the Placental Mammals having 
retained the same set as the lowlier and earlier Marsupials. 
* Of course, as Mr. Bateson has shown (in his paper read before the 
Zoological Society on Feb. 2—not yet published), one may easily attempt 
to carry this principle of the individual homologization of teeth too far, as 
no doubt in my efforts to find a nomenclature by which we could name 
each Marsupial tooth I have myself done in my catalogue of that order. 
Still, without entering into this question before the publication of his 
paper, I may claim that the above is by no means a straining of the true 
principles of tooth homology. 
One possibility, however, would take away the value of the above 
suggestion, namely if it were shown that neither Zrconodon nor any of 
the other 4-premolared Mesozoic mammals were marsupials at all; but 
they have been considered as such by all paleontologists, and the changing 
of the last premolariform tooth is certainly not an argument against their 
being so. 
+ The close resemblance of mp.* to the molars beth in form and struc- 
ture has already suggested this homology to several observers, although it 
has hitherto usually been explained by the adaptive necessity for a 
grinding-tooth at the back of the tooth-row during youth. 
