744 MAMMALIA. 
Australasia. But fossil remains found in Europe and 
America show that they once had a wide range. As 
there are no higher Mammals indisputably indigenous to 
Australasia, it seems as if the insulation of that region had 
occurred after the Marsupials had gained possession, but 
before higher mammalian competitors had arrived. Thus 
saved and insulated, the Marsupials have evolved in many 
different directions. 
The brain is less developed than in Eutherian Mammals, 
for the convolutions are simple or absent, the anterior com- 
missure is large, the corpus callosum is practically absent. 
In the skeleton there are several peculiarities: thus the 
angle of the lower jaw is more or less inflected, except in 
the genus Zursipes; the jugal reaches far back to share in 
making the glenoid cavity ; there is practically only one set 
of teeth; there are more incisors above than below (except 
in the wombat), and the number of incisors sometimes 
exceeds three on each side. There are usually epipubic or 
marsupial bones in front of the pubic symphysis. These 
have no connection with the marsupium, as is evident from 
the fact that they occur in both sexes; they are sesamoids 
developed in the inner tendon of the external oblique 
muscle of the abdomen. 
The teeth cannot be readily reduced to the typical Eutherian formula. 
According to recent research, the milk set is degenerate, and is usually 
represented only by the last premolar, which in most cases cuts the 
gum, and is for a time functional. The other teeth correspond to 
the permanent set of the Eutheria. According to another view, the 
functional teeth are milk-teeth. In living Marsupials there seems to 
be a suppression of what in typical placentals would be called the 
second premolar. 
A common sphincter muscle surrounds the anus and the 
urogenital aperture, and in the majority of cases the 
anus lies so much within the urogenital sinus that the 
arrangement may be described as cloacal. The scrotal sac 
containing the testes lies in front of the penis—a unique 
position. The genital ducts of the females are often 
separate throughout, so that there are two uteri and two 
vaginze. But the bent proximal parts of the vagine some- 
times fuse and form a caecum, which, according to the 
degree of fusion, may be a single tube or divided by a 
