An infraclass of therian mammals including a single order, the Marsupialia. The Metatheria are distinguished from the Eutheria (the placental mammals) by numerous characters. The full metatherian dentition is I 5/4 C 1/1 Pm 3/3 M 4/4, for a total of 50 teeth. The braincase is small, the angular process of the mandible is inflected, and a pair of marsupial bones articulates with the pelvis. Almost all living marsupials have a pouch on the belly of the female in which the young are carried after birth. The Metatheria arose from unknown therians in the Cretaceous or earlier at about the same time as eutherian mammals. For a time the two groups evolved side by side, but the marsupials were unable to compete with the more progressive later placental forms and died out except in South America and Australia, where they were isolated by water barriers.