Placental mammals that gestate their young internally, as do we and most of the pelts we know, account for 95 percent of the world’s 5,600 species of mammals, and marsupials like the kangaroo, koala and our very own opossum, whose young are born at a grublike stage and do most of their developing externally, often in a pouch, constitute nearly all the rest.
Source: wiktionary