442 J. T. WILSON AND J. P. HILL. 
of any other tooth of the adult dentition. This early forma- 
tion of the deciduous premolar is not peculiar to Perameles. 
It is important to note that it occurs also in Didelphys. 
But its precocity cannot be explained in Perameles, as it 
usually has been in Didelphys, by a reference to the future 
size of the tooth, since, as is well known, it isin Perameles 
by far the smallest of the whole cheek-tooth series, and, in- 
deed, is absolutely a small-sized tooth. Neither can the early 
development be explained by its position in the jaw. Indeed, 
from its relatively posterior position in the young and develop- 
ing jaw, quite the contrary, i.e. a retardation, might have 
been expected, such as affects the hinder (molar) elements of 
the dental series. The discussion of this important matter 
will be resumed further on in this paper. So much it was 
essential to state at the outset to render our position in- 
telligible. 
It was only with great reluctance that we arrived at this 
apparently inevitable conclusion that the deciduous premolar 
was a member of the “ prelacteal” series, and although it 
seemed impossible to interpret the facts in the case of Pera- 
meles in any other way, it seemed doubtful whether we 
should be able to reconcile this view satisfactorily with the 
facts observed in the case of other Marsupials, more particu- 
larly in Didelphys. At least, it seemed difficult to do so in 
a way that would carry conviction to those whose views on the 
marsupial dentition were moulded upon the conditions repre- 
sented in Didelphys. In that animal it has indeed been 
shown (cf. Kikenthal [7] ) that the deciduous premolar is, at 
the first, the most advanced of all the teeth; but the subse- 
quent course of its development, resulting in the production 
of a relatively large molariform tooth, seemed anything but 
favorable to a theory according to which it must be regarded 
as belonging to an otherwise degenerated “ prelacteal ” order 
of teeth. 
It may now appear somewhat curious that the very obvious 
possible explanation of the whole difficulty did not dawn upon 
us at the very outset. But so completely were we under the 
