92 
MR. OWEN ON THE STRUCTURE OF 
masses of the hemispheres in the Beaver and all other placentally developed Mam- 
malia, and which exists in addition to the hippocampal commissure, is wanting in the 
brain of the Wombat : and as the same deficiency exists in the brain of the Great 
and Bush Kangaroos, the Vulpine Phalanger, the Ursine and Mauge’s Dasyures, and 
the Virginian Opossum, it is most probably the characteristic of the marsupial divi- 
sion of Mammalia. 
In the modification of the commissural apparatus above described, the Marsupialia 
present a structure of brain which is intermediate to that of the placental Mammalia 
and Birds, in which class the great commissure is wholly wanting, and the hemi- 
spheres, though comparatively larger than in many of the Mammalia, are brought 
into communication only by means of the anterior, posterior, and soft commissures, 
and a slight trace of the fornix or hippocampal commissure. 
Of the other peculiarities of the marsupial brain, the relatively large size of the 
anterior commissure is most worthy of notice ; its development is correspondent 
with the large size of the cerebral ganglion, which forms the chief origin of the olfac- 
tory nerve, and some of the anterior fibres arch forwards, and are directly continued 
into those nerves. 
In the position, superficial transverse fissure, and solidity of the bigeminal bodies, 
the marsupial brain adheres to the mammiferous type, as also in the exterior trans- 
verse fibres of the commissure of the cerebellum, forming the pons Varolii, the presence 
of which is in relation with the development of the lateral lobes of the cerebellum. 
Other minor points of difference between the brains of the Marsupiata themselves 
will be explained in the description of the figures. 
Meanwhile their agreement in so important a modification of the cerebral organ as 
the absence of a corpus callosum and septum lucidum, affords additional and strong 
grounds for regarding the Marsupiata as a distinct and peculiar group of Mammalia ; 
and when to this modification of cerebral structure are added the traces of the ovi- 
parous type of structure presented in the circulating and absorbent systems, together 
with the peculiarities of the osseous and generative apparatus, we may with reason 
suspect that distribution of the Marsupiata to be artificial and founded on an imper- 
fect knowledge of their mutual affinities which, from a modification of the teeth and 
extremities alone, would separate and disperse the species amongst corresponding 
groups of the placental Mammalia. 
Cuvier has observed that the marsupial group of quadrupeds embraces forms 
which typify different orders of the ordinary Mammalia and M. De Blainville 
regards them as forming, with the Monotremata, a division apart from the placental 
Mammalia. The metropolis of this subclass is the continent of Australasia, where 
the different carnivorous, insectivorous, omnivorous, and herbivorous genera act 
* “ Les Marsupiaux — nous paraissent devoir former un ordre a part, tant ils offrent de singularity dans leur 
economie, et surtout parceque l’on observe en quelque sorte la representation de trois ordres bien differents.” — 
R&gne Anim. i. p. 172. 
