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.
The short and broad teeth terminate in four subequal toes, protected by short rounded hoofs, and all reaching the ground.
In the true cavies, or couies, Cavia, the foreand hind-limbs are short and of subequal length, the ears are short and there is no tail.
The Old World porcupines, constituting the family Hystricidae, are terrestrial, stoutly built rodents, with limbs of subequal length in front and behind, and the skin covered with strong spines.
A remarkable feature of the belt is the longitudinal chain of broad valleys - the Great Appalachian Valley - which, in the southerly sections divides the mountain system into two subequal portions, but in the northernmost lies west of all the ranges possessing typical Appalachian features, and separates them from the Adirondack group. The mountain system has no axis of dominating altitudes, but in every portion the summits rise to rather uniform heights, and, especially in the central section, the various ridges and intermontane valleys have the same trend as the system itself.
Fore-feet with five distinct subequal toes with claws.
The three bones of the first row of the carpus (scaphoid, lunar and cuneiform) are subequal in size.