436 Ju T. WILSON AND J.P. Hib, 
the Marsupials have suffered loss of all the teeth of the “ per- 
mauent” series in front of the last premolar, with, indeed, the 
doubtful exception, according to Rose, of the last upper incisor 
in some forms. 
In this connection Rése remarks upon the “ noteworthy con- 
stancy of the group with reference to the single tooth-change 
from Mesozoic up to modern times—a constancy which had 
seemed to Thomas so inexplicable,—and he offers the following 
explanation :—“ Die Beuteltiere sind aber bei der Reduktion des 
vielfachen Zahnwechsels der reptilienahnlichen Vorfahren der 
heutigen Sauger gleichsam iiber das Ziel hinausgeschossen und 
haben sich in eine Sackgasse verrannt, aus der kein Ruickweg 
moglich ist” (1, p. 705). He also suggests that such a reduc- 
tion could only be serviceable if at the same time the milk- 
teeth came to grow from persistent pulps,—a condition which, 
he remarks, is attained only by the wombat. 
Rose agrees with Kikenthal in referring the molars to the 
same series as the other teeth, i.e. to the milk or first denti- 
tion, if they are to be referred specifically to one or other of 
the two series represented by the antemolar teeth. Into his 
later view respecting the possibility of the molars representing 
*end-members of separate dentitions’’ we need not now enter. 
In 1893 a highly interesting and important paper was pub- 
lished by Rose (11) on the subject of tooth development in 
Phascolomys. We are of opinion that the full significance 
of the observations therein recorded has been apprehended 
neither by subsequent writers nor by the author himself, though 
the latter undoubtedly attached considerable importance to 
them. 
The results of Roése’s research (for which unfortunately only 
one pouch specimen, of 19 mm. body-length, was available) 
would seem to tend towards a confirmation of a long discredited 
statement of Owen’s (15), to the effect that the incisor teeth 
and the first molar are changed in the young animal. It 
appears from Rose’s investigation that at a comparatively early 
1 This view regarding the nature of the last polyprotodont incisor has 
received no confirmation whatever from any later observer. 
