MARSUPIALIA. 
portance. in the structure of the Wombat, 
except this solitary character of the Marsupium, 
to separate it from the Rodent order ?”—a ques - 
tion which he might in 1831 have asked with 
equal force in reference to any other Marsupial 
genus,—could only be answered satisfactorily 
by the submission of the Marsupialia in ques- 
tion to a thorough dissection. 
Although the Marsupials present modifica- 
tions of the dental system corresponding with 
the carnivorous, omnivorous, and herbivorous 
types, yet they agree with each other, and 
differ from the analogous Placental Mammalia 
in having four instead of three true molars, 
i.e. four molars which are not displaced and 
succeeded by others in the vertical direction. 
The incisor teeth, also, either exceed in number 
those of the analogous Placental classes, or are 
peculiarly arranged and opposed to each other. 
In the locomotive organs it is true that we 
see some of the Marsupials having a hinder 
thumb, like the Placental Quadrumana; others 
are digitigrade, with falculate claws, like the 
Placental Fere; a third, as the Wombat, has 
the feet adapted for burrowing; a fourth, like 
the Cheironectes, is aquatic, and has webbed 
feet ; yet all these Marsupials agree with each 
other in having a rotatory movement of the 
hind foot, analogous to the pronation and su- 
pination which, in the F pepo quadrupeds, 
are limited when enjoyed at all to the fore feet ; 
and they manifest moreover a peculiar modi- 
fication of the muscles of the hind leg and foot 
in relation to these rotatory movements. In 
those Marsupials, as the Kangaroos, Potoroos, 
and Perameles, in which the offices of support 
and locomotion are devolved exclusively or 
in great part upon the hind legs, these are 
strengthened at the expense of the loss of the 
rotatory movements of the feet; but in the 
‘enormous development of the two outer toes, 
and the conversion of the two inner ones into 
-unguiculate appendages, useful only in cleans~ 
ing the fur, these Marsupials differ from all 
Placentals, whilst the same peculiar condition 
‘of the toes may be traced through the Pedima- 
nous group of Marsupials. Thus the locomo- 
tive organs, notwithstanding their adaptation 
to different kinds of progression, testify to the 
unity of the Marsupial group in the two 
remarkable peculiarities of structure above 
The vascular system gives evidence to the 
same effect. We have seen that the Marsupials 
present the following peculiarities in the struc- 
ture of the heart: viz. the right auricle mani- 
fests no trace of either fossa ovalis or annulus 
ovalis, and receives the two vene cave supe- 
riores by two separate inlets. This genera- 
lization is, however, less urgent than the pre- 
ceding in the present question, because the 
modification, as regards the separate entry of 
the superior vene cave, obtains in a few pla- 
cental species, as in the elephant and certain 
Rodents; but as the first cited cardiac cha- 
racter is common and peculiar to the Mar- 
supial Mammalia, and as the second, while 
it is universal in the Marsupials, occurs only 
as an exceptional condition in fhe placental 
329 
series, the arguments which they afford to the 
unity of the Marsupial group cannot be over- 
looked ina philosophical consideration of the 
affinities of the Mammalia. 
With respect to the nervous system, it has 
been shown that in the structure of the brain, 
the Marsupialia exhibit a close correspondence 
with the Ovipara in the rudimental state of the 
corpus callosum ; the difference which the most 
closely analogous placental species offer in this 
respect is broadly marked. 
hese coincidences in the Marsupialia of 
important organic modifications of the dental, 
locomotive, vascular, cerebral, and reproductive 
systems, establish the fact, that they constitute, 
with the Monotremes, a natural group inferior 
on the whole in organization to the Placental 
Mammalia. 
The following is a tabular view* of the subor- 
dinate divisions in the Marsupialia regarded as 
an order of the Implacental sub-class of Mam- 
malia :— 
* Of the various forms of Marsupial animals 
attempted to be arranged in natural groups in the 
present classification, it may be asked which is the 
typical form? or in other words, which genus com- 
bines most of the points of organization peculiarly 
characterizing the Marsupialia ? 
We have seen that certain modifications of the 
nervous, circulating, and generative systems are 
common to all the genera. But the female gene- 
rative organs approach nearest to the Rodent 
type in the small dorsigerous Opossums, in which 
the characteristic external pouch becomes very 
nearly obsolete. It is not the genus Didelphys 
therefore that we should select as the type of the 
Marsupials. It appears to me that there is both 
a dental and a digital character which may be re 
garded as eminently marsupial; the former, be~ 
sides the number of true molar teeth, consists in 
the opposition of six vertical incisors above toa 
large procumbent single pair below; the latter is 
exemplified in the atrophied and coadunate con- 
dition of the second and third digits of the hinder 
foot. The Phalangers, Petaurists, Koalas, Kan- 
garoos, and Potoroos possess, in addition to the ordi- 
nary Marsupial characters, both these modifications 
of teeth and digits. It seems, therefore, that it is 
from one of these genera that we should select the 
Marsupial type par excellence. If we say the Pha- 
langers, it may be objected that the hinder hand 
and opposable prehensile thumb indicate in these 
a transition from the Marsupialia to the Quadru- 
mana. Should the Petauri be our choice, then 
again we perceive in the development of the lateral 
tegumentary folds, and their connection with the lo- 
comotive members, a tendency to the Flying Squir- 
rels, The tail-less Koala may be deemed to ex- 
hibit in its clumsy form and proportions a resem- 
blance to the tree-bears, The Kangaroos and 
Potoroos obviously typify the Rodent Jerboas, and 
they have lost the peculiar rotation of the hind leg 
and the muscular modification connected there- 
with. If, however, the type of a natural group of 
animals, and such I have proved the Marsupial 
group to be, is that which manifests the greatest 
number of the structural modifications peculiar to 
the group, and the smallest number of such as are 
common to other natural assemblages of Mammalia, 
then the Koala has the best claim to typical pre- 
eminence. The Marsupial bones might be readily 
supposed to afford a simple indication of the most 
peculiarly Marsupia! animal, if they offered different 
relative magnitudes in different genera: now the 
range of variety in this respect is, in fact, consi- 
derable, and the Marsupial bones present their 
greatest development in the Koala. 
