period of uterine gestation, and the consequent 
non-enlargement- of the abdomen, their presence 
might be expected. In the female they assist in 
producing a compression of the mammary gland, 
necessary for the alimentation of a peculiarly 
feeble offspring, and they detend the abdominal 
viscera from the pressure of the young, as these 
increase in size during their mammary or mar- 
supial existence, and still more when they after- 
wards return to the pouch for temporary shelter.” 
It is without doubt in the mammalia that the 
brain is perfected. <c We can trace through the 
different orders the increasing complication of 
this organ, until we find it in man to have 
attained that condition which so eminently dis- 
tinguishes him from the rest of the class ; and 
if the introduction of new powers into an 
organism requires a modification in its mode of 
development, w r ith what other than the perfec- 
tion of the nervous system can we connect true 
viviparous or placental generation ? for we do 
not perceive that in their digestion, circulation, j 
respiration, locomotion, or temperature, the 
mammiferous vertebrata are in any degree ad- 1 
vanced beyond the bird, in consequence of their 
more complex, or, as it may be termed, more 
careful generation.” According to this view, 
Professor Owen undertook a careful examination 
of the brain of various marsupial animals, and 
the result was that besides the decreased size of 
the hemispheres of the brain and consequent 
exposure of the cerebellum, indicative of a low 
grade of organisation, the corpus callosum and 
spetum lucidum were found to be entirely 
wanting or at least existing only in a 
rudimentary state. The corpus callosum has 
been considered as the great characteristic of the 
brain in the mammalia, and the w T ant of it 
in this order of mammals, coupled with the 
ovoviviparous generation of the marsupialia will, 
I think, warrant the placing of this order as the 
lowest organism in the class mammalia. The 
corpus callosum, which is the principal bond of 
union between the opposite hemispheres of the 
brain, is regarded as the great characteristic of the 
brain in the mammals, and in fact this commis- 
sural apparatus presents the essential difference 
which exists between that and the oviparous 
vertebrata classes. There is also a remarkable 
feature in the skull of the marsupials, which 
consists in the permanent separation of the 
greater portions of the bones ; they do not 
anchylose in the adult individuals (as do most of 
the bones of the skulls in the placental series), 
the temporal bone generally presents a perma- 
nent separation of the squamous, petrous, and 
tympatic elements. “ I have observed,” says 
Professor 0\Ven, c< this reptile-like condition ol 
the bone in the mature skulls of the marsupials.” 
The palatine portion of the skull is very imper- 
fect, presenting large openings which are wanting 
in placental mammals ; there is also a peculiarity 
in the lower jaw in all the species of the marsu- 
pialia, with the exception of the Echidna and 
Ornithorhyncus — “ the angle of the lower jaw is 
as if it were bent inwards in the form of a pro- 
cess encroaching in various shapes and degrees of 
development in the different marsupial genera 
upon the interspace of the rami of the lower jaw. 
In looking down upon the lower margin of the 
lower jaw we see, therefore, in place of the mar- 
gin of a vertical plate of bone, a more or less 
flattened surface extending between the external 
ridge and the internal process or inflected angle.” 
One of the most striking peculiarities in the 
marsupial animals consists in the premature 
birth of their young, aud' consequently, the im- 
perfect state of development which they present 
at this period compared with other animals. The 
period of utero-gestation varies considerably in 
the different animals of this group. On the 
macropas major, or large grey kangaroo, it is 
about thirty-nine days, after which time it con- 
tinues its foetal life in the pouch for ten or 
twelve weeks : the manner of conveying the em- 
bryo animal to the nipple, has not been fully 
ascertained, but I believe the mouth is the vehicle 
used. Professor Owen examined the young of 
the species above named twelve hours after birth: 
“ It resembled an earthworm in the color, and 
semi-transparency of its integuments, adhered 
firmly to the point of the nipple, breathed strongly 
but slowly, and moved its fore legs when dis- 
turbed. Its body was bent upon the abdomen, 
its short tail tucked in between the hind legs, 
which were one-third shorter than the fore legs, 
but with the three divisions of the toe distinct ; 
the whole length from the nose to the end of the 
tail when stretched out, did not exceed one-inch 
and one-sixth.” Experiments have been 
made on a foetus of three weeks old 
by detaching it from the nipple, and after 
an hour’s separation, it, on being held to the 
nipple, regained its hold, and sustained no injury. 
When the foetus is very young, it requires some 
little force to remove it, and in doing so an in- 
jury to the young animal might probably be 
done. Until it attains a certain age the pouch 
of the mother has its orifice closed, and as it 
were glued to the body of the parent by a pecu- 
liar secretion ; as the young acquires strength 
this secretion disappears, and the young leave 
the pouch to return at will. They attain a large 
size before they entirely quit their retreat j and 
