SUCCESSION OF THE TEETH IN THE DASYURID2E. 
453 
hitherto unnoticed, but most striking and peculiar, retardation in the development of 
the first upper incisors in all the three Polyprotodont families, these families being the 
very ones in which, as the incisors are not so specialised in another direction as are 
•those of the Diprotodonts, we should most expect to find traces of the development 
of milk incisors. In half-grown specimens, such as those of Sarcophilus ur sinus and 
Phctscologale wallacei (figured Plate 27, figs. 11 and 12), at a time when the three outer 
incisors are fully up and in use, the first pair, the largest in the adult (Ph. wallacei, 
fig. 13), are still quite minute, with their points only just projecting above the bone, 
and altogether in a very marked condition of retardation. And the same appears to be 
the case in young specimens of Didelphyidse, Peramelidae, and other Dasyuridee, in 
many of which the first incisors in the fully adult animal are by far the largest of all. 
The theory now suggested, therefore, about these Marsupials with retarded first 
incisors, is that they represent at the present day the stage at about which the 
common ancestors of the Metatheria and Eutheria diverged from each other—a stage 
when the teeth were, as it were, preparing themselves for the assumption of milk 
predecessors, a process which has in the latter group been continued onwards until 
the complete Diphyodont condition has been attained. On the other hand, in the 
former the process has, except in the case of pm 4 , never gone beyond the initial stage 
of the retardation of i 1 , a stage which has itself been continued owing to its own 
inherent value. 
For this view as to the first incisors it may also be urged that the most likely of all 
the teeth to undergo a marked alteration of any sort would be those at the extreme 
ends of the series, judging by the manner in which, throughout the Mammalia, the 
first incisor and the last molar show themselves plastic in readily taking on characters 
not, or only much later, found in the neighbouring teeth. 
Before leaving this subject, I would wish to point out that it is by no means essen¬ 
tial to the general views here advocated that this suggestion as to the first incisors 
should be correct, but only that, as some tooth or other must have been the first to 
follow the example of pm 4 in developing a milk predecessor, the first incisor, even 
apart from its retardation, is at least as likely as any other to have been that tooth, 
while on this theory we also gain a possible explanation of the same very curious 
retardation. Should, however, future palaeontological research show that any other 
tooth, say the canine or pm 3 , took on a milk predecessor before i 1 , it would only 
disprove the present suggestion without in any w r ay invalidating the general con¬ 
clusions come to. 
In order now to put into order the various suggestions above made, and to utilise 
them for the purpose of making out the past history of tooth evolution, we will 
commence by drawing up diagrams on the same principle as in fig. 1, but the whole 
set of teeth, instead of only the premolars, is taken into account (fig. 2).* Here IV. 
'* In these diagrams and their explanations the teeth of the upper’ jaw only are referred to, as it is 
there alone that, owing to the presence of the premaxillo-maxillary suture, the true relations of the 
