The infraclass of therian mammal species in which females bear their young in an immature condition (while still in the early fetal stage) and then permit their further infant development in an abdominal pouch covering their mammary glands. Compared to the delayed births of placental mammals, this is inefficient in keeping young infants alive. Metatherian mammals are also called marsupials. Included in this infraclass are kangaroos, koalas, opossums, etc.