This sub-class contains the single order Marsupialia with a large number of families, one the Didelphidae, confined to America, the remainder to the Australian and Austro-Malayan sub-regions. The auditory bulla is formed by a process of the alisphenoid and the tympanic is a loose semiring. The carotid canals pierce the basi-sphenoid as in Cetacea, among Eutheria, and as in Sauropsida. The palate often has unossified vacuities and the angle of the lower jaw is inflected except in Tarsipes. The median inferior piece of the atlas vertebra may remain unanchylosed (Thylacinus), or be absent altogether (Phascolomys, Macropus, &c). There are as a rule two epipubic or 'marsupial' bones. In the brain the corpus callosum is small and the anterior commissure large. The retinal cones contain coloured oil globules. The crowns of the teeth conform to very different patterns according to the habit of life, i.e. carnivorous, insectivorous, rodent, etc. There may be more than three incisors on each side in the upper, rarely in the lower, jaw, and the number in the upper and lower is equal only in Phascolomys, which has two above and below.

There are three praemolars and four molars above and below on each side, reversing the numbers usual in Eutheria. The last praemolar alone has a milk predecessor, and it is not certain that this is the case in all forms. The dentinal tubes are frequently continued on into the enamel. The character of the stomach varies much with the food. The anus opens in the female, except in Kangaroos, just within the sinus urogenitalis, forming a rudimentary cloaca, and both orifices are in all cases surrounded by a common sphincter. The fossa ovalis is absent in the heart, and the musculi papillares arise in the right ventricle only from the septum. The testes are suspended in a scrotum in front of the penis. The glans penis is bifurcate except in Kangaroos, and the crura of the corpora cavernosa are either free altogether from the ischia, as in most instances, or are attached to the symphysis pubis by ligamentous fibres (Macropus, Hypsiprymnus), or have the usual connection as in a Phascogale (Sack). The left ovary is sometimes the larger of the two, and the ripe Graafian follicles project from the surface of the ovary to a degree rarely observable in Eutheria. The oviducts are differentiated into oviducal, uterine and vaginal sections which are typically separate throughout their whole extent, and open separately into the urogenital sinus which is of considerable length.

But in some instances the proximal portions of the vaginae fuse and develope a median caecum extending towards the urogenital sinus. The cavity of this caecum is divided by a complete septum, each half being continuous with the vagina of its own side in the Wombat (Phasolomys). The septum may be lost, and in a few instances (e. g. Macropus Bennetti) the caecum opens direct into the urogenital sinus. The yolk sac, or umbilical vesicle, is large, fused to a limited area of the subzonal membrane, and is variously stated to be vascular (Osborne) or non-vascular (Caldwell). The allantois is small and vascular, but it is doubtful whether it always fuses to the subzonal membrane: if so, union occurs at a late period. The subzonal membrane is attached to the uterine walls either by villi or by villiform pseudopodial processes of the external cells (Caldwell) which are developed only from the region covered by the yolk sac. The uterine glands enlarge during pregnancy, and perhaps secrete a nutritive fluid.

Intra-uterine life is brief, extending from 2 weeks (Opossum) to 38 days (Macropus major). The number of teats which are abdominal in position vary from 4 to a larger number in mutiparous forms, e.g. 13 in the Virginian Opossum. In this case they are grouped round a central teat and not extended in lines. The marsupial pouch for the young enclosing the teats is rudimentary, or absent in some Opossums (Didelphidae). Its aperture is generally directed forwards, but backwards in Thylacinus, Perameles, and Choeropus. In the Kangaroo milk is forced down the throat of the young animal by the contraction of the cremaster muscle covering the mammary gland.

The Mammalian remains, consisting chiefly of isolated teeth and lower jaws, small in size, from Triassic and Jurassic strata in England and America, are generally referred to this sub-class. Cf. Owen, 'British Fossil Mammals and Birds,' 1846, and Palaeontographical Society's Publications, 1871; Marsh, American Journal of Science, xx. 1880, p. 235. Fossil post-tertiary Marsupialia, some gigantic in size, occur in Australia. Cf. Owen's Memoirs in Ph. Tr., republished in two vols. 1877.

Paratherium, an extinct genus of Didelphidae, occurs in European Eocene and upper Miocene strata.

Waterhouse, Marsupiata, 'Natural History of Mammalia,' i. London, 1846; Gould, 'Mammals of Australia,' 3 vols. London, 1845-63.

Corpus callosum, Flower, Ph. Tr. 155, 1865. Tongue of Marsupialia, Poulton, P. Z. S. 1883, and Q. J. M. xxiii. 1883. Union of crura penis to pelvis. Sack, Z. A. ix. 1886. Female genitalia. Brass, Inaugural Dissertation, Leipzig, 1880, cf. Zeitschr. f. ges. Naturw. liii. 1880, p. 672. Ovum, Poulton, Q. J. M. xxiv. 1884. Foetal membranes, Osborne, Q. J. M. xxiii. 1883; Caldwell, Ibid. xxiv. 1884.