292 Dr. A. Fleischmann on the Relationship 
been somewhat changed by the secondary influence of the 
musculature there inserted ; it does not prevail, however, 
throughout the whole group, and is always absent in the 
Hystrichidse, Subungulata, Octodontidse, Lagostomidse, and 
Leporidge. 
This modification, however, may be referred back to con- 
ditions within the Marsupial series, for among them many 
forms have lost a distinct mandibular angle, such as, for 
example, Phascolarctos. Then the lower jaw, if looked at 
from the side, appears as a band dilated posteriorly into a 
triangular plate. Nevertheless the contour of the margin and 
the pits and bony ridges occurring on the outer surface of the 
end of the jaw betray the previous history of the part by very 
intelligible tokens. Even in true Marsupials we find evidence 
of the endeavour to bring the mandibular angle from the 
inwardly directed horizontal position into a more vertical one 
and into the same plane as the ascending branch. In Rodents 
all desirable steps of the retroversion have been retained, in 
the end giving origin to the great increase of the surface of 
the posterior extremity of the mandible. 
Side by side with this we recognize a reduction of the coro- 
noid process ; very strongly developed in the Marsupials, it 
is retained in all the Rodents which possess an inwardly pro- 
jecting mandibular angle, but it becomes small until it nearly 
disappears in Rodents with a broad mandibular plate. 
As I conceive the origin of the dentition of the Rodentia to 
have passed through stages such as the living survivors of 
the leaping and climbing Marsupials still display in model, 
the dentition of their ancestors must have gradually lost the 
omnivorous character and become herbivorous; consequently 
the direction of movement of the lower jaw must also at the 
same time have become modified. 
In point of fact this transformation may be still recognized 
from the position and form of the condi/lus glenoidalis in the 
lower jaw, which passes from the transverse direction general 
in the omnivorous Marsupials into a position parallel to the 
sagittal plane ; and, in accordance with this, the cavitas gle- 
noidalis on the squamose part of the temporal, which in the 
Marsupials attains no great extension, becomes gradually 
longer so as to pass on to the jugal arch and become a long 
groove-like excavation. 
The occurrence of the change of food may be further in- 
ferred from the constitution of the digestive organs in the 
Rodentia. I indicate now only the form and structure of the 
stomach. Whilst in most Rodents this possesses a pretty 
simple structure and form, it becomes more highly compli- 
