No. 412.] IHE AUSTRALIAN MARSUPIALIA. 25I 
group as the descendants of a common stock, the Didelphyidze 
are so primitive that it is scarcely possible to regard them, as 
a family, otherwise than ancestral.! 
THE EVOLUTION OF THE TEETH IN AUSTRALIAN MARSUPIALS. 
It has already been mentioned that progressive evolution in 
dentition, in the case of the placental mammals, is accompanied 
by a reduction of certain of the teeth and by an elaboration of 
certain others, notably the molars. The researches of Cope, 
Osborn, and others have shown that in the evolution of the 
molar teeth the elaboration proceeds from a trituberculate con- 
dition in the upper series, and from a quinquetuberculate or 
sexituberculate condition in the lower series. These types are 
exemplified in the teeth of the primitive Creodonta of the early 
Eocene (Protochriacus, Oxyclzenus, etc.). 
1 This involves the question of the limitations of the family Didelphyide. 
Lydekker ('96, p. 109), while assigning the didelphyid incisor formula to the 
ancestors of the oe pin (p. 55) of the Didelphyide and Dasyuride 
as descendants of a comm ock. Spencer ('96, p. 188, Summary) speaks of 
on the one hand, and to the early Australian t on the other. agito 
(98, p. 16) regards the bai called by him rr uti as ancestral to the 
Didelphyide and to the tralian Dasyuride. In the writer's opinion it will 
not be found advisable Shans to regard the Didelphyidz as a modern derived 
group, but that, when the actual ancestors of the Australian forms are identified, 
it will probably be found necessary to extend the family Didelphyidz to include 
the : f 
ch 
about the time when the Australian fauna probably arose. Second, d 
ily Didelphyidz, as at present limited, is not equivalent to any one of the four 
Rinit families of Australian marsupials, the latter being much more comprehen- 
sive. The diversity of organization which is met with among the Dasyuride in 
the inii Phascologale and Thylacinus, eue the Phalangeride in the genera 
Phalanger, Phascolarctos, and Tarsipes, among the Macropodide in the genera 
Hypsiprymnodon and Macropus, is absolutely phi parallel among the Didel- 
phyide. If, therefore, the integrity of the Australian families is to be retained, — 
and there is every reason for r believing that they are natural groups, —there are 
good grounds for extending the family Didelphyidz to include their own ances- 
tors and those of the Australian forms, if the latter, as they probably did, pos- 
Sessed the main didelphyid modifications of the teeth and feet, even while 
differing in Prensa minor details. The Microbiotheriidae of Ameghino may be a 
case in poi 
