The class or sub-class Implacentalia,' of which the Marsupialia form one order, also in cludes a second order, the Monotreznata, which can only be termed Didelphes ' in the sense in which the word is applicable to many of M. de Blainville's ' Monodelphes,' i. e. in re ference to their having two distinct uterine tubes. But the merit of the primary division of the Mammalia into PLACENTALIA and Ihr PLACENTALLA does not rest upon the appro priateness of the terms, but upon the esta blishment, by a long series of anatomical re searches, of a primary division of the Mam miferous class, which before was a mere hy pothesis.
Many acute and sound-thinking naturalists refused their assent to the views of Cuvier and De Blainville, which, as they were supported by a knowledge of the conformity of organiza tion of only the generative system in the Mar supials, were unquestionably defective in the evidence essential to enforce conviction. The best arguments for returning to the older views of classification, and for distributing the Mar supial genera, according to the affinities appa rently indicated by their dental and locomotive systems, among the different orders of the Placen tal Mammalia, have been advanced by Mr. Ben nett, the accomplished author of the Gardens and Menagerie of the Zoological Society de lineated, (vol. i. p. 265) ; and these have been repeated with approbation, and adopted by later systematists, as by Mr. Swainson.
The discovery of the true affinities of the Marsupialia could only flow from an insight into their whole organization, and the question which Mr. Bennett proposes with reference to the genus Phascolomys, " What is there of im 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 Ferw; 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; 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 Pedirna 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 ovalis or annulus ovalis, and receives the two vent 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 yam cav, 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 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