Discoveries in Mammalian Dentition. 309 
advanced in reduction ; and (II.) that Mammals were primi- 
tively monophyodont and that the milk-dentition was super- 
added as a secondary development, the development being 
naturally most advanced in the highest orders. 
The latter view was adopted and carried out in great 
detail by myself *, and therefore now that Dr. Kiikenthal’s 
discoveries have shed a new light on the subject Iam impelled 
to express the revised opinion that they have induced me to 
form. 
The second of the two theories referred to had as its primary 
basis the nearly complete monophyodontism of the Marsu- 
pials, and the moment these were proved to have been ever 
more largely diphyodont than they are at present the whole 
case would fall to the ground. And such proof seems now 
to have been found by Dr, Kiikenthal in the nearly complete 
set of rudimentary successional teeth discovered by him in 
embryos of Didelphys; which can hardly be interpreted 
otherwise than he has done, namely as rudiments of a 
previously functional second set of teeth. 
Such being the case I am now for my own part prepared 
to admit that Mammals must have been originally diphyodont 
and that their regular diphyodontism was probably in direct 
succession to the irregular polyphyodontism of their Reptilian 
ancestors, or may even have existed in what were in other 
respects members of the latter class. 
At the same time it is evident that on this view many of 
the known facts seem to become more instead of less difficult 
of interpretation. Thus the fact that Tréconodon, one of the 
earliest known Mammalia, changed a single tooth only +, 
and that the very one which changes in the modern Marsu- 
pials, now appears most inexplicable, and is alone almost 
calculated to stagger belief in primitive diphyodontism. 
This problem, however, may be left for time to unravel, 
but its existence is sufficient to excuse those who, before 
these latest discoveries were made, could not bring them- 
selves to believe in that view of the ancestral history of 
Mammalian teeth. 
The same fact, combined with the presence of four un- 
doubted premolars (of whichever series”) in so many of 
the earliest Marsupials, renders it also difficult, if not impos- 
* Phil. Trans. 1887, p. 443. 
t The specimen of Yriconodon (Triacanthodon) figured in my paper 
has, by the kind permission of Dr. Woodward, been carefully developed 
beneath all the cheek-teeth, and reexamined by the light of Dr. Kiiken- 
thal’s discoveries. No other successional teeth, however, besides that 
below p.* are present in the jaw. 
