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.
Hind feet with the four outer toes sub-equal, with claws similar to those in the fore feet; the first toe almost always distinct and partially opposable, though small and nailless, sometimes absent.
The tail is rudimentary, the first hind-toe opposable, the first pair of upper incisors very large, but the second and third either absent or small and placed partially behind the larger pair; and only five pairs of cheek-teeth in each jaw.
Fore-feet with the two inner toes slightly separated from and opposable to the remaining three, all with strong curved and much compressed claws.
- Skeleton of Right incisor very large and chisel-like, Hind-Foot of Koala (Phas- molars with prominent transverse colarctus cinereus), showing ridges, as in Macropus, but without stout opposable hallux, folthe longitudinal connecting ridge.
Hind-feet short and broad, with five welldeveloped toes; the first large, nailless and opposable; the second and third slender and united by a common integument as far as.
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.
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.
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 Javan Pithechirus has the thumb opposable, while the Papuan Chiruromys has the tip of the tail naked above and prehensile.
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.
Man differs from them in the absence of a hairy coat; in the development of a large lobule to the external ear; in his fully erect attitude; in his flattened foot with the non-opposable great toe; in the straight limb-bones; in the wider pelvis; in the marked sigmoid flexure of his spine; in the perfection of the muscular movements of the arm; in the delicacy of hand; in the smallness of the canine teeth and other dental peculiarities; in the development of a chin; and in the small size of his jaws compared to the relatively great size of the cranium.
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.
The hand of the gorilla corresponds essentially as to bones and muscles with that of man, but is clumsier and heavier; its thumb is " opposable " like a human thumb, that is, it can easily meet with its extremity the extremities of the other fingers, thus possessing a character which does much to make the human hand so admirable an instrument; but the gorilla's thumb is proportionately shorter than man's.
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.
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.