How are placental mammals classified?

Placental mammals are grouped into Xenartha (anteaters, sloths, and armadillos), Afrotheria (elephant shrews, tenrecs, golden moles, elephants, aardvarks, manatees, dugongs, and hyraxes), Euarchontoglires (rodents, rabbits, hares, pikas, tree shrews, colugos, and primates), and Laurasiatheria (hedgehogs, shrews, moles, solenodons, bats, whales, dolphins, porpoises, even-toed hoofed mammals, odd-toed hoofed mammals, pangolins, and carnivores). Each of these is further divided into orders, which are divided into families, which are divided into genera, which are divided into species. There are 21 orders and 132 families of placental mammals.