OF THE MAKSUPIALIA AND MONOTEEMATA. 
649 
sented by the upper and anterior part of the transverse band passing between the hemi- 
spheres of the marsupial brain 1 The most important and indeed crucial test in deter- 
mining this question, is its position in regard to the septum ventriculorum, and especially 
the precommissural fibres of the fornix. Without any doubt in all marsupial and 
monotreme animals examined (sufficient to enable us to affirm without much hesitation 
that it is the character common to all) it lies above them, as distinctly seen in the trans- 
verse sections. Moreover, passing outwards into the hemispheres, it overarches or forms 
the roof of the lateral ventricles of the cerebrum. This is precisely the same relation- 
ship as that which occurs in Man and all other mammalia. 
The defective proportions of the part representing the great transverse commissure 
of the placental mammal, which appears to me to result from, or, at all events, to be 
related to the peculiar conformation of the wall of the hemisphere, must not lead to 
the inference that the great medullary masses of the two halves of the cerebrum are by 
any means “disconnected.” The want of the upper fibres is compensated for in a 
remarkable manner by the immense size of the anterior commissure, the fibres of which 
are seen radiating into all parts of the interior of the hemisphere. There can be little 
doubt but that the development of this commissure is, in a certain measure, comple- 
mentary to that of the corpus callosum. That it is not simply correspondent with the 
large size of the olfactory ganglion, as Professor Owen has suggested, is shown by the 
fact that in the Hedgehog and some other placental mammals this ganglion attains a 
far greater proportionate volume than in many marsupials, and yet the commissure is 
very considerably smaller. 
In descending the series from Man to the Placental Mammals of lowest cerebral 
organization, the great change in the condition of the corpus callosum has been seen to 
be, the disappearance of the rostral portion, and the coincident greater development of 
the posterior folded or psalterial portion ; the latter being connected with the relative 
increase of the hippocampal region of the cerebrum. In the brain of the marsupial a 
change of precisely the same nature is carried to an excess. There is, however, as far as 
my observations show, no structure characteristic of the higher group which is absent in 
the lower. 
The step from the marsupial or monotreme brain to that of an animal belonging to 
one of the lower vertebrate classes is very great. Indeed it is difficult to see in many of 
the peculiarities of their brain even an approach in the direction of that of the bird. 
We may allow that the diminution of the volume of the corpus callosum leads on to its 
entire absence ; but in the great development of the anterior commissure is presented a 
special characteristic of the lowest group of mammalia, most remarkable because it is 
entirely lost in the next step of descent in the vertebrate classes. The same may be 
said of the cerebral folding constituting the hippocampus major. 
Plate XXXVI. figs. 5 & 6 are views of the brain of a Goose, corresponding to those 
given of the various mammals. The smooth, thin, inner wall has no trace of that folding 
upon itself which gives rise to the hippocampus major in the mammal. In this respect 
mdccclxv. 4 u 
