T. H. Huxley, Dollo and R. A. Bensley have adduced much evidence tending to show that the common opossum (Didelphys virginiana) embodies nearly all the characters one might reason ably expect to find in the remote common ancestors of all the higher marsupials, since it retains all their primitive characters but has assumed few specializations of its own. This is not equivalent to saying, however, that American opossums are ancestral to the Australian marsupials. It probably means only that the American opossums are the survivors of an ancient and diversified group, which in some earlier geological epoch was spread perhaps as far east as Asia and which gave rise on the one hand to the American marsupials and on the other to the Australian carnivorous mar supials and later groups.
The tree-climbing habits of the opossum are expressed in its muscular prehensile tail and especially in the grasping power of its hands and feet, each with five fully developed toes ; the "fric tion pads" on the sole of the foot also are well developed. The forearm can be freely twisted on the upper arm and the collar bones (clavicles) are well developed, as in most climbing mam mals. In the hind-limbs the great toe is set off widely from the others, the feet are plantigrade and capable of being freely turned inward.
The braincase of a large opossum is relatively small and unex panded in keeping with its low type of brain. The muzzle is large and in its interior the delicate bones that support the mem branes of the sense of smell are well-developed ; the bony orbits or sockets for the eyes are rather small, as are the eyes themselves; the inner ear-bones are relatively small and the same is true of the resonating chamber of the middle ear. All these and many similar facts suggest that the opossum is not an intelligent ani mal, that it pursues its prey 'chiefly by the sense of smell, and depends for safety upon hiding in the daytime and prowling about at night.
In conformity with its carnivorous-omnivorous habits, the skull of a large opossum recalls those of other carnivorous mammals in its stout cheek arches and in its high median and occipital crest, all for the support of the stout jaw muscles. The dentition also tells plainly that the opossum is primarily a flesh-eating animal, which however has not acquired highly specialized shearing teeth like those of a cat but has retained certain primitive features. The general outline of an upper molar crown of an opossum is that of a scalene triangle with the apex on the inner side, the shorter leg being in front, the longer one behind. From the general surface
of this triangle two small, V-shaped cusps project : a very small one in front (the paracone) and a much larger one behind (the metacone). Between any two adjacent upper molars are empty embrasures or openings, each like an inverted V with the tip pointing outward. Into these embrasures fit the elevated V-shaped forepart of the lower molars, surmounted by three blade-like cusps, the apex of this V pointing outward. The inverted V of a lower molar shears past the large V of an upper molar ; while the inwardly projecting apex of an upper molar fits into a concave projection constituting the rear half of a lower molar. Thus the upper and lower molars of the opossum present fundamentally the same combination of shearing V's and piercing projections and sockets which is to be found in the most primitive mammals of the Cretaceous and Eocene ages.
In the opossum, as in all primitive marsupials, there are on each side four upper molars and four lower, also three upper premolars and three lower premolars. The upper premolar crowns have a single high tip, equivalent to the highest cusp on the crown of the molars. Thus there are in all seven teeth on each side, above and below, behind the canines. The same number of postcanine teeth is to be found in primitive placental mammals, but in the latter there are four premolars and three molars. The canine teeth, both upper and lower, of the opossum have long-curved, sharp-tipped crowns, well fitted for fighting and killing. The incisor teeth are small and numerous with simple crowns, five on each side in the upper jaw and four in the lower jaw; of these the first upper incisors are somewhat larger and have downwardly directed points, while the first lower incisors are somewhat inclined forward. Here then is the first hint of the nipping or prehensile development of the incisors which becomes pronounced in the diprotodont group of marsupials. Accordingly the dental formula, showing the num ber of different kinds of teeth in the upper and lower jaws of the common opossum may be written as follows : (Incisors 4 Canines Premolars 4 Molars 4) X 2= 50. From this as a starting-point the dental formulae of all other marsupials appear to have been derived either by reduction and loss of teeth or, in the case of Myrmecobius, by the secondary increase in number of the molar teeth. As in all typical marsupials, only the hinder or third pre molars of the adult dentition are preceded by deciduous teeth.