310 Mr. O. Thomas— .Vo^es on Dr. W. KukenthaVs 



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 tlie 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 furtiier, and especially palfeontological, evidence 

 for its solution ; but comparing the dentition of Triconodon 

 with tliose 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 Triconodon are 

 liomologous with the four premolars of the typical Placental 

 dentition*; and, as a consequence, (o) that the changing 

 premolar of Marsupials is homologous with p."* of Placental 

 Mammals. 



But if once the primitive diphyodont theory be admitted, 

 the homologization of the Marsupial molars with the milk 

 series is 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. '1 — not vet published ), one may easily attempt 

 to carry this principle of the individual homologization of teeth too far, as 

 no doubt in my eflbrts to find a nomenclature by which we could name 

 each Marsujiial 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, nanielj,' if it were shown that neitlier Triconodon nor any of 

 the other 4-premolared Mesozoic mammals were marsupials at all ; but 

 they have been considered as such by all pakeontologists, and the changing 

 of the last premolariform tooth is certainly not an argument against their 

 being so. 



t The close resemblance of mp.* to the molars both 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 r 

 grinding-tooth at the back of tne tooth-row during youth. 



