OF THE MABSUPIALIA AND MOXOTREMATA. 
647 
sure in the Kangaroo, are also represented by it, though in a reduced degree, in the 
Echidna. Perhaps the greatest change is in the extreme reduction of the septum, as 
best seen in the transverse section (Plate XXXVIII. fig. 8). In dissecting the brain 
from above, the fibres of the superior commissure are found to spread out into a delicate 
layer roofing in the ventricles quite to the anterior part of the hemisphere, as described 
in the Kangaroo. 
Having described the actual condition of an important and well-marked region of the 
cerebrum in several members of the two great groups of the Mammalia, it now remains 
to trace out the relation that the several structures entering into the formation of this 
region bear to one another in each of the two groups. It will be necessary also to 
inquire how far the results brought out by the present method of examination are in 
accordance with the views generally received. 
At the outset a distinct confirmation is afforded by the dissections recorded in this 
paper, of the great fact, first observed by Professor Owen, that the brains of animals of 
the orders Marsupialia and Monotremata present certain special and peculiar characters, 
by which they may be at once distinguished from those of other mammals. The appear- 
ance of either a transverse or longitudinal section would leave no doubt whatever as to 
which group the brain belonged. In the differentiating characters to be enumerated, 
some members of the higher section present a considerable approximation to the lower ; 
but, as far as is known at present, there is still an interval between them unconnected 
by any intermediate link. 
The differences are manifold, but all have a certain relation to, and even a partial 
dependence on, each other. 
They may be enumerated under the following heads : — 
1. The peculiar arrangement of the folding of the inner wall of the cerebral hemi- 
sphere. A deep fissure, with corresponding projection within, is continued forwards 
from the hippocampal fissure, almost the whole length of the inner wall. In other 
words, the hippocampus major, instead of being confined as it is, at least in the higher 
forms of placental mammals, to the middle or descending cornu of the lateral ventricle, 
extends up into the body of the ventricle, constituting its inner wall. 
2. The altered relation (consequent upon this disposition of the inner wall) and the 
very small development of the upper transverse commissural fibres (corpus callosum). 
3. The great increase fin amount, and probably in function, of the inferior set of 
transverse commissural fibres (anterior commissure). 
These propositions must now be considered a little more closely. Arguing from our 
knowledge of the development of the brain in placental mammals (for of that of the 
marsupials we have at present no information), it may be supposed that the first- 
named is also first in order of time in the gradual evolution of the cerebral structures. 
Before any trace of the budding out of the fibres which shoot across the chasm sepa- 
rating the two hollow sac-like hemispheres, before the differentiation of a portion of the 
