New Latin, from Latinmarsupium, marsuppium (“pouch, purse”), from Ancient Greek μαρσύπιον (marsupion) or μαρσύππιον (marsuppion), variants of μαρσίππιον (marsippion), diminutive of μάρσιππος (marsippos, “bag, pouch”), + -alia.
English Wiktionary. Available under CC-BY-SA license.
In elevating the marsupials to the rank of a sub-class the name Metatheria has been suggested as the title for the higher grade, with Marsupialia as the designation for the single order by which they are now represented.
It is, however, less liable to cause confusion, and in many other ways more convenient to employ the better known term Marsupialia in both senses.
Thomas, Catalogue of Monotremata and Marsupialia in the British Museum (1888); "On Caenolestes, a Survivor of the Epanorthidae," Proc. Zool.
Bensley, "A Theory of the Origin and Evolution of the Australian Marsupialia," American Naturalist (1901); "On the Evolution of the Australian Marsupialia, &c.," Trans.
P. Hill, "Contributions to the Morphology of the Female Urino-genital Organs in Marsupialia," Proc. Linn.