This sub-class contains all the orders of existing Mammalia except two. Its general characters are those typical of Mammalia, from which the two other sub-classes deviate in certain respects. The distinctive features of the reproductive system and of development are as follows. The scrotum, when present, is behind the penis. The uteri either remain distinct throughout their entire length (uterus duplex), e. g. Rabbits; or fuse distally having a common aperture into the vagina, but retaining an internal septum in the fused portion (uterus bipartitus), e. g. most Rodentia and many Chiroptera; or they fuse throughout the greater part of their whole length having a common cavity, and leaving only the oviducal portions of the tube distinct (uterus bicornis), the commonest form of all (Ungulata, Carni-vora, etc.); or finally the uteri are completely fused (uterus simplex) as in Man and Apes. The vagina is always single, and opens, as a rule, into a vestibule or urogenital sinus common to it and the urethra, except where the clitoris is perforated by the latter.
The yolk sac, or umbilical vesicle, is large in Rodentia, Insectivora, and Chiroptera, and fuses with that part of the subzonal membrane left free by the allantois; in other Eutheria it is small, and does not reach the subzonal membrane. The allantois always fuses with a greater or less proportion of the subzonal membrane, and renders it vascular, forming the foetal placenta. This placenta is in relation with the uterine walls which become thickened and more vascular during pregnancy, forming the maternal placenta. The foetal placenta is furnished with vascular villi, which are either simply in contact with corresponding depressions in the maternal placenta, or else fuse with that structure. If at the birth of the young the maternal placenta persists, or only loses epithelium, the placenta is said to be non-deciduate; if vascular parts of it come away also, the placenta is said to be deciduate. The non-deciduate placenta is either diffuse when the villi are scattered (most Ungulata, except Ruminantia; Sirenia, Cetacea, Lemuridae, among Primates); or cotyle-donary, when they are aggregated into patches corresponding with maternal patches (true Ruminantia). The deciduate placenta is either discoidal when the villi are developed over a circular area and form with the maternal structures a cake-like mass (Rodentia, Insectivora, Chiroptera); metadis-coidal when the villi, at first scattered, are restricted to a limited area, as in the placenta of Man and Simiidae; or zonary when the villi are restricted to a partial or complete girdle surrounding the embryo (Hyrax, Elephant, Carnivora). The Edentata have different types of placenta: it is non-deciduate and diffuse in Manis; deciduate and zonary in Orycteropus, and Dasypus novem-cinctus, or discoidal in the remainder.
In Man and Simiidae the ovum sinks at an early stage into the maternal mucous membrane, which completely surrounds it, or is reflected over it. Traces of a similar reflection are observable in some zonary placentae.
The nine orders of living Eutheria are separated from one another by subordinate characters. They are (1) the Edentata (Sloths, Ant-eaters, Armadillos in America, Cape Ant-eater in South Africa, Pangolins (Manis) in Africa and the Oriental region); (2) the Sirenia, - Manatee from the rivers flowing into the Atlantic in Africa and South America: Dugong (Halicore) from the Red Sea, East of Africa, Ceylon, Bay of Bengal, Indo-Malayan Archipelago, North coast of Australia; (3) the Cetacea, including toothed and whale-bone Whales; (4) the Insectivora, a group of small inconspicuous Mammals with a wide distribution; (5) the Chiroptera or Bats; (6) the Rodentia, an order widely and evenly distributed with a large number of families; (7) the Ungulata, an order comprising a large number of dissimilar groups, including the Hyracoidea, Proboscidea (Elephants), the Perissodactyla (Horse, Rhinoceros, Tapir), the Artiodactyla, subdivisible into the Ruminantia (Cows, Sheep, Goats, Antelopes, Deer, Camels, Giraffe), and the non-Ruminantia, i. e. the Hippopotamus and Suidae; (8) the Carnivora, including a sub-order Fissipedia with the groups Aeluroidea (Cat-group), Cynoidea (Dogs), and Arctoidea (Bears, Weasels, &c), as well as a sub-order Pinnipedia, or the Seals; (9) the Primates, which includes the Lemuridae, Simiidae (Monkeys and Apes), and the Hominidae (Anthropidae) or Man.
There are many extinct groups, especially groups allied to Ungulata. For these, see Marsh's papers in the American Journal of Science; his Deinocerata in the United States Geological Survey, x. 1884; Cope, 'Vertebrata of Tertiary formations of the West,' Report of U. S. Geological Survey of Territories, iii. 1885; his papers in the American Naturalist for the last few years; Flower, 'Mammalia' (supra)-, Wallace, 'Distribution of Animals,' 1876, caps. vi. vii. viii; Schlosser, 'Stammesgeschichte der Huftthiere,' etc. M.J. xii. (1), 1886. Tritylodon longaevus, a Triassic South African Mammal of doubtful affinities, Owen, Journal Geol. Soc. 40, 1884.