In liftenmalia, however, in most of the orders of which the connexion of the ovum with the uterus is so much more intimate than in the preceding classes, the variations in the structure of the female sexual organs are more numerous and remarkable ; and though it be admitted that the nature of the foetal coverings and ap pendages results from the original constitution and properties of the ovum, yet the modifica tions of the uterus have evidently, in this class, a subordinate relation to those differences.
In tracing the female generative apparatus from the human subject through the different orders of Manunalia, we find that it approxi mates to the oviparous type of structure in two ways, viz., by an obliteration of the os finer; which is the characteristic limit between the uterus and the vagina in this class ; and by a gradually increasing division of the uterus and vagina until they become two separate tubes throughout their entire extent.
In no mammiferous genus do the female organs present that character of unity or con centration, with distinction of parts, which is found in the human subject ; for in the lower orders, besides the more essential differences above-mentioned, there is always an elongation of the uterus, with a thinning of its parietes, and in general a blending together of the urethral and sexual passages. This latter deviation com mences in the and in the Lentures the angles of the uterus begin to elongate and to assume the form of cerium. The mesial cleft increases, and the cornua preponderate in the Carnivore, the Cetaraer, the Runduantia, and the Pachydertnata ; but it is in the Rodentia, which present affinities to Birds in other parts of their structure, that the uterus is first found completely divided into two lateral halves. This structure is not, indeed, uniformly met with in all the genera of the order; but besides the Hare and Rabbit, in which the double uterus is allowed to exist by De Graaf and Cuvier, a similarly complete division of the organ obtains in the genera Sciurus, Arctoutys, Spalax, Bathgeigus, Echintys, Eretizon, (F. Cuvier) and Hydrocherus ; while in the genera .Mus, Cattle, Ccelogenys, and Dasvprocta a portion of the true uterus still remains undi vided ; though this part, to which alone the term corpus uteri' can be properly applied, is ex tremely small or rudimental. Nevertheless,
although the corpus uteri exists in these genera, the true vagina is as remarkable for its length and capacity as in those in which the corpus uteri has ceased to exist.
Hitherto the vagina has presented itself un der the form of a simple undivided canal, communicating with the urethro-sexual passage, at least after impregnation by a single aperture. But it is a remarkable and interesting fact that in the Sloth, in the Mare and Ass, in the Pig, and in the Cow, the vagina in the virgin state com municates with the urethro-sexual passage by a double aperture, in consequence of 'being traversed by a narrow vertical septum or chord. This septum has been described by veterinary authors as a hymen in the Mare ; the analo gous part in the human subject also occasionally presents the same structure, and has even been observed in some cases to extend as a mesial partition inwards towards the uterus.
In the Marsupialia, where from the small size of the foetus at birth a similar conformation is permitted to remain as a permanent structure, the vagina is in some genera wholly, and in others partially divided ; but the divided por tion in the latter is always that which is nearest the urethro-scxual passage.
The true uterus is completely divided in all the Marsupial genera, and each division is of a simple elongated form, as in the Rodentia.
superadded complications in the female generative organs of the Marsupials, as com pared with other mammals, are not then rightly attributable to the uterus, but to the vagina ; and they are of such a nature as to adapt the latter to detain the flatus, after it has been ex pelled from the uterus, for a longer period than in other 31ammalia.
These complications vary considerably in the different marsupial genera. On a comparison of the female organs in Didelphys dorsigera, Petaurus pygnurus, and Petaurus taguanoides, in Dasyurus viverrinus, in Didelphys Virgi sdana, in Miteropus major, and Ilypsiptyinaus marines, I find that the relative capacity which the uteri bear to the vagina diminishes in the order in which the above-named species follow, while the size of the external pouch increases in the same ratio.