MARSUPIALIA. 



320 



portance in the structure of the Wombat, 

 except this solitary character of theMarsupium, 

 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 Ferae; 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 placental 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 ; hut 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 

 cited. 



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 avails or annulus 

 ovalis, and receives the two vents cav<e 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 venre cavse, 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 the placental 



series, the arguments which they afford to the 

 unity of the Marsupial group cannot be over- 

 looked in a 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. 



These 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 digiial 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 to a 

 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- 

 langes, 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 Marsupial 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. 



