250 THE AMERICAN NATURALIST. [Vor. XXXV. 
As regards the modifications of the teeth and feet, there- 
fore, the Didelphyidz exhibit characters which are ancestral 
to those of the Australian marsupials. The question now 
arises as to whether or not they are ancestral in other respects. 
The opportunity has not yet presented itself to the writer of 
examining the case in detail, so that only a few examples will 
be given here. As was pointed out in 1880 by Brass, the com- 
plex modifications which result in the formation of the peculiar 
median vagina in specialized marsupials are seen in their incip- 
ient stages in the Didelphyide, where the two vaginz bend 
toward one another in the middle line. Passing onwards from 
the Didelphyidz every gradation is met with, until the final 
stage is reached in the Macropodide, 
Again, taking the case of the marsupium, the anteriorly 
directed opening of this structure, as seen in the Phalangeridz 
and Macropodidz, and the posteriorly directed opening, as seen 
in the Peramelidae and Notoryctida, are obvious departures 
from such a type as is seen in the Didelphyidze and Dasyuride, 
in which the opening is directed vertically downwards. 
Finally, taking the case of the tail, which is an adaptive 
structure of the greatest importance in marsupials, it is entirely 
probable that the hairy, non-prehensile condition, where it 
occurs in the Australian forms, has been derived from a naked, 
scaly, prehensile one, such as is seen in the Didelphyidz. 
Thomas (88) describes for the Dasyuridz, in which the tail is 
hairy and never prehensile, a scaly condition in various genera, 
among them Phascologale wallacei, P. flavipes, and Sminthopsis 
leucopus. A scaly tail, which, according to Thomas, is said to 
be also prehensile, is found in Perameles broadbenti among the 
Peramelida. Every grade of prehensilism is met with among 
the Phalangeridz ; and, finally, among the Macropodide, a scaly 
tail is found in Hypsiprymnodon moschatus, and a prehensile 
one in the various species of Bettongia. 
The above facts, when added to the evidence of the teeth 
and feet, suggest strongly a former connection of the Didel- 
phyide with the Australian marsupials, and although, in 
attempting to express such a connection phylogenetically, it 
is a natural procedure to regard the original members of each 
