- The third and last sub-order of marsupials is the Diprotodontia, which is exclusively Australasian and includes the wombats, koala, cuscuses, kangaroos and their relatives.
By means of the little musk-kangaroo, the cuscuses and phalangers, constituting the family Phalangeridae, are so closely connected with the kangaroos, or Macropodidae, that in the opinion of some naturalists they ought all to be included in a single family, with three sub-families.
Cuscuses and phalangers form a numerous group, all the members of which are arboreal, and some of which are provided with lateral expansions of skin enabling them to glide from tree to tree like flying-squirrels.
The typical members of the group are the cuscuses (Phalanger), ranging from the Moluccas and Celebes to New Guinea, in which the males are often different in colour from the females.
Although intimately connected with the cuscuses and phalangers by means of the musk-kangaroo, the kangaroos and wallabies, together with the rat-kangaroos, are easily distinguishable from other diprotodont marsupials by their general conformation, and by peculiarities in the structure of their limbs, teeth and other organs.
The marsupials also attain their maximum development in Australia (" Notogaea " of the distributionists), extending, however, as far west as Celebes and the Moluccas, although in these islands they form an insignificant minority among an extensive placental fauna, being represented only by the cuscuses (Phalanger), a group unknown in either Papua or Australia.