362 NATURAL EY SCTENGE. May, 
Marsupials branched off from the main Mammalian stock and 
obviated their difficulties in other ways than by the development of a 
complete temporary dentition. 
At the time of propounding this lucid and carefully elaborated 
theory, Mr. Oldfield Thomas pointed out that it would entirely collapse 
should a more complete replacement be discovered among any of the 
Marsupials, and he has been the first to acknowledge that this has 
now been done by Kikenthal. 
Years ago Professor Huxley stated his belief that the deciduous 
dentition now found in Marsupials was but a relic of a fuller milk 
dentition, instancing an analogous loss in the case of the shrew, 
which has no deciduous teeth, while its near ally, the hedgehog, has. 
a complete replacement. At the time, this belief seemed barely 
tenable, since in no Marsupial had any trace of a larger number of 
deciduous teeth been found, while even the one present in most was 
known to be absent in some forms (Phascogale apicalis and others). 
Moreover, fossil Marsupials, such as Tviconodon, show the replace- 
ment of the posterior premolar, and of that tooth only. This, how- 
ever, only affords one more instance of the danger of generalising 
from purely negative results, for Huxley’s belief has now been amply 
justified by Kiikenthal’s discoveries, and diphyodontism is proved to 
have been the ancestral condition of at least the Didelphyde. 
It is, of course, possible that a complete cycle of development has 
been undergone, the milk dentition at first appearing as a provisional 
masticatory apparatus and afterwards persisting while their phylo- 
genetic predecessors underwent abortion. There is, however, little or 
no foundation for such a suggestion; a far simpler explanation is to 
regard both series as representatives of two consecutive series of the 
Reptilian teeth. 
In making this supposition, we must, moreover, grant some proba- 
bility that the true molars were at one time subject to replacement, and 
this hypothesis also receives confirmation from the recent researches. 
on the Marsupials. 
Kiikenthal appears to have been first led to work at the em- 
bryonic dentition of the Marsupials from the consideration of the 
synchronous development of the deciduous posterior premolar and 
the remaining permanent premolars. This suggested the idea that 
all the premolar teeth belonged to the same series, and Kikenthal 
has succeeded in proving that this is the case by the discovery of 
rudiments of an almost complete series of successional teeth. 
These embryonic rudiments were found, not only for the incisors 
and canines, but also for the most anterior premolar, and for the first 
and second teeth of the true molar series. The only rudiments not found 
were, therefore, those of the second premolar and of the more posterior 
molar teeth. Further, these vestigial rudiments of all the suc- 
cessional teeth are identical in structure with the developing third 
premolar, which is afterwards cut, replacing its deciduous predecessor. 
