Class Mammalia

Mammals are divided into 3 sub-classes based on their mode of reproduction:
Subclass Prototheria
Prototheria is composed of egg-laying mammals. There are only 6 species of these and they belong to one order:

•  Monotremata (platypus and echidna)

Subclass Metatheria
Metatheria numbers to 250 species and is composed of seven orders (previously 1 - Marsupialia). Their young are born tiny and immature and must climb into the mother's pouch, where they grab hold of a teat and do not leave until they are mature.

•  Didelphimorphia (New World opossums)
•  Paucituberculata (South American rat opossums)
•  Microbiotheria (colocolo)
•  Dasyuromorphia (dasyurids, thylacines) 
•  Peramelemorphia (bandicoots)
•  Notoryctemorphia (marsupial moles)
•  Diprotodontia (kangaroos, koalas, wombats, possums)

Subclass Eutheria
The sub-class Eutheria is composed of all the placental mammals, whose young form as embryos in the mother's stomach. Humans are an example of the sub-class Eutheria. Below is a list of all 19 orders in this sub-class:

•  Insectivora (moles, shrews)
•  Dermoptera (flying lemurs)
•  Chiroptera (bats)
•  Cetacea (whales)
•  Carnivora (cats, bears, dogs, otters, seals, sea lions)
•  Tubulidentata (aardvarks)
•  Proboscidea (elephants)
•  Hyracoidea (hyraxes)
•  Primates (monkeys, lemurs, bushbabies, aye-ayes)
•  Xenarthra or Edentata (armadillos, anteaters, sloths)
•  Pholidota (pangolins)
•  Lagomorpha (rabbits, hares, pikas)
•  Rodentia (mice, rats, squirrels, porcupines, beavers, voles, hamsters)
•  Sirenia (manatees, dugongs)
•  Perissodactyla (horses, donkeys, zebras, rhinoceroses,tapirs)
•  Artiodactyla (pronghorns, deer, camels, gnus, goats, giraffes, hippopotami, pigs, peccaries, chevrotains, musk-deer, cows)
•  Scandentia (tree shrews)
•  Macroscelidea (Elephant Shrews)