MAMMALS

A Different Class: Mammals are separated from other animals by over 300 million years of evolution. During that time, several traits evolved that distinguish them from other vertebrates — namely hair, sweat glands, mammary glands, a modified middle ear and an advanced brain. In mammals, two bones that were once part of the jaw became part of the middle ear; so, a mammal's middle ear has three bones to aid in hearing instead of just one.

Separated by Birth: There are around 5,400 species of mammals divided into two main categories: those that lay eggs (monotremes) and those that bear live young (marsupials and placentals). There are only five species of monotreme: the platypus and four species of echidna. Marsupials, which carry their young outside the womb through early infancy, account for 334 species, including roughly half of all Australian mammals (kangaroos, koalas, wombats, etc.) as well as some New World animals, like opossums. All other mammals — rabbits, rodents, shrews, moles, anteaters, sloths, armadillos, badgers, pangolins, bats, primates, whales, elephants, manatees, camels, cattle, deer, antelope, goats, etc. — are placental. Their young are nourished in the womb by a placenta, which delivers nutrients to the developing fetus.

http://animal.discovery.com/mammals/