322 PROF. OWEN ON THE CLASSIFICATION 
In the Didelphys Yapock,—the type of the subgenus Cheironectes,—the anterior ex- 
tremities, besides being web-footed, present an unusual development of the pisiform 
bone, which supports a fold of the skin, like a sixth digit ; it has indeed been described, 
as such, by M. Temminck ; this process has not of course any nail. The dentition of 
the Yapock resembles that of the ordinary Didelphys. All the Opossums have the inner 
digit of the hind foot converted by its position and development into a thumb, but with- 
out a claw. The hinder hand is associated in almost all the species with a scaly pre- 
hensile tail. 
In some of the smaller Opossums the subabdominal tegumentary folds are rudimental, 
or merely serve to conceal the nipples, and are not developed into a pouch ; the young 
in these species adhere to the mother by entwining their little prehensile tails around 
hers, and cling to the fur of the back ; hence the term dorsigera applied to one of these 
Opossums’. 
Tribe IIl. CARPOPHAGA. 
Stomach simple ; cecum very long. 
In this family, the teeth, especially those at the anterior part of the mouth, present 
considerable deviations from the previously described formule ; the chief of which is a 
predominating size of the two anterior incisors, both in the upper and lower jaws. 
Hitherto we have seen that the dentition in every genus has participated more or less 
of a carnivorous character ; henceforth it will manifest a tendency to the Rodent type. 
The Phalangers, so called from the phalanges of the second and third digits of the 
hinder extremities being inclosed in acommon sheath of integument, have the innermost 
digit modified, to answer the purposes of a thumb ; and the hinder hand being associated 
in many of the species with a prehensile tail, they evidently, of all Frugiwora, come near- 
est to the arboreal species of the preceding section. In a system framed on locomotive 
characters they would rank in the same section with the Opossums. We have seen, 
however, that they differ from those Entomophagous Marsupials greatly in the condition 
of the intestinal tube. Let us examine to what extent the dental characters deviate 
from those of the Opossums. 
In the skull of a Phalangista Cookii, now before me, there are both in the upper and 
lower jaw four true molars on each side, each beset with four three-sided pyramidal 
sharp-pointed cusps ; thus these essential and most constant teeth correspond in number 
with those of the Opossum: but in the upper jaw they differ in the absence of the in- 
ternal cusp, which gives a triangular figure to the grinding surface of the molars in 
1 Few facts would be more interesting in the present branch of zoology than the condition of the new-born 
young, and their degree and mode of uterine development in these Opossums. Since the marsupial bones serve 
not, as is usually described, to support a pouch, but to aid in the function of the mammary glands and ¢estes, 
they of course are present in the skeleton of these small pouchless Opossums, as in the more typical Marsupials. 
ro 
