Placentals
Placental mammals; an infraclass of viviparous mammals with embryos nourished by a placenta. The young are born in an advanced stage of development. This is the result of an efficient nutritive connection between mother and embryo. The embryos are nourished by a placenta in which the membranes of the chorion and allantois play an essential part in carrying food materials and oxygen to the embryo and wastes and carbon dioxide back to the mother's blood stream. This is established by the fingerlike villi which project from the embryonic portion of the placenta into the uterine portion.
Eutherian females have a single vagina of which the two uteri can be unfused to form duplex uteri (found in marsupials, in many rodents and bats) and to form a bipartite uterus or distally fused to form a bicornuate uterus, or even completely fused as in the higher mammals to form a simplex uterus.
A marsupial pouch is never present.
The penis is not bifurcated, and a baculum is present in many forms. A scrotum may be present or absent; when present, it is located posterior to the penis in all forms except the order Lagomorpha, in which it is located anterior to the penis. The brain is small to large, and a corpus callosum is present.
There is a high variability in the dentition of the eutherians; the incisors never number more than 3/3, and frequently occur in equal numbers in the upper and lower jaws. The primitive dental formula consists of: incisors 3/3, canines 1/1, premolars 4/4, and molars 3/3, for a total of 44. There are no epipubic bones.
Eutherian mammals occur on all continents, in all oceans, and on many oceanic islands.
Seventeen living Eutherian orders are recognized: Insectivora, divers group of small eutherian mammals; Dermoptera, Colugas or flying lemurs; Chiroptera, Bats; Primates, prosimians, tarsiers, monkeys, apes and man; Edentata, sloths, armadillos and anteaters; Pholidota, Pangolins or scaly anteaters; Mysticeta, baleen whales; Odontoceta, toothed whales; Carnivora, carnivores; Lagomorpha, rabbits, hares and pikas; Rodentia, rodents; Tubulidentata, aardvark; Proboscidea, elephants; Hyracoidea, hyraxes, conies or dassies; Sirenia, manatee, dugong and sea cow; Perissodactyla, odd-toed ungulates: horses, rhinoceroses and tapirs; Artiodactyla, even-toed or cloven-hoofed ungulates.
Of these, Insectivora is often split into 2 or more orders. Carnivora was long considered to be 2 orders: Carnivora, containing only the fissiped carnivores, and Pinnipedia, containing the seals, sea lions, and walruses. Mysticeta and Odontoceta are considered by many authors to be only suborders of a single whale order (Cetacea).
(after DeBlase, 1982)