In characters of such importance as the structure of the hand and foot, the lower apes diverge extremely from the gorilla; thus the thumb ceases to be opposable in the American monkeys, and in the marmosets is directed forwards, and armed with a curved claw like the other digits, the great toe in these latter being insignificant in proportion.
The foot resembles that of the other lemurs in its large opposable great toe with a flat nail; but all the other toes have pointed compressed claws.
Perhaps, however, the most interesting member of the whole group is the tiny musk-kangaroo (Hypsiprymnodon moschatus) of north-east Australia, which alone represents the sub-family Hypsiprymnodontinae, characterized by the presence of an opposable first toe on the hind-foot and the outward inclination of the penultimate upper premolar, as well by the small and feeble claws.
Fore-feet with the two inner toes slightly separated from and opposable to the remaining three, all with strong curved and much compressed claws.
Hind-feet rather long and slender, with a well-developed opposable and nailless first toe;: second and third digits united, with sharp, compressed curved claws; the fourth and fifth free, with small flat nails.
They differ in certain respects, as in the proportion of the limbs, in the bony development of the eyebrow ridges, and in the opposable great toe, which fits the foot to be a climbing and grasping organ.
These ancestral mammals, in addition to their small size, were characterized by the presence of five toes to each foot, of which the first was more or less completely opposable to the other four.
The hollow tympanic bullae, they have the clavicles imperfect, the first front toe opposable to the rest, the temporal region of the skull roofed with bone, and the crowns of the molars with cusps arranged in rows but eventually covered by a layer of enamel.
The first family is that of the true or American opossums- Didelphyidae, in which there are five pairs of upper incisors, while the feet are of the presumed primitive arboreal type, the hind foot having the four outer toes subequal and separate, with the first opposable to them all.