324 PROF. OWEN ON THE CLASSIFICATION 
Genus Petaurus. 
There are many species of Marsupials limited to Australia, and closely resembling or 
identical with the true Phalangers in their dental characters and the structure of the 
feet. I allude to the Petaurists or Flying Opossums ; these, however, present an exter- 
nal character so easily recognizable, and influencing so materially the locomotive facul- 
ties, as to claim for it more consideration than the modifications of the digits or spurious 
molars, which we have just been considering in the Phalangiste. A fold of the skin 
is extended on each side of the body between the fore and hind legs, which, when out- 
stretched, forms a lateral wing or parachute, but which, when the legs are in the posi- 
tion for ordinary support or progression, is drawn close to the side of the animal by the 
elasticity of the subcutaneous cellular membrane, and then forms a tegumentary ridge. 
These delicate and beautiful Marsupials have been separated generically from the other 
Marsupials under the name of Petaurus': they further differ from the Phalangers in 
wanting the prehensile character of the tail, which in some species of Petaurus has a 
general clothing of long and soft hairs, whilst in others the hairs are arranged in two 
lateral series. 
Now in the Petaurists there is as little constancy in the exact formula of the dentition 
as among the Phalangers. The largest species of Petaurus, Pet. Taguanoides, e. g., is 
almost identical in this respect with the Phalangista Cook, which M. Fr. Cuvier has 
therefore classed with the Petauri. Those teeth of Pet. Taguanoides, which are sufii- 
ciently developed, and so equal in length, as to exercise the function of grinders,—or in 
other words, the functional series of molars,—include six teeth on each side of the upper 
jaw, and five teeth on each side of the lower jaw. The four posterior molars in each 
row are true, and bear four pyramidal cusps, excepting the last tooth in the upper jaw, 
which, as in Phal. Cookii, has only three cusps. In the upper jaw, the space between 
the functional false molars and the incisors is occupied by two simple rudimentary teeth, 
the anterior representing the canine, but being relatively smaller than in Phal. Cooki. 
The crowns of the two anterior incisors are relatively larger. In the lower jaw the 
sloping alveolar surface between the functional molars and large procumbent incisors 
is occupied, according to M. Fr. Cuvier, by two rudimentary minute teeth: I have not 
found any trace of these in the two skulls of Pet. Taguanoides examined by me. In 
Phal. Cookii there are three minute teeth in the corresponding space, but these differ- 
ences would not be sufficient ground to separate generically the two species if they were 
unaccompanied by modifications of other parts of the body. In Petaurus sciureus and 
Petaurus flaviventer the dentition more nearly resembles that of Phalangista vulpina. 
In the upper jaw the functional molar series consists of five teeth on each side, the four 
hinder ones being, as in Pet. Taguanoides, true tuberculate molares, but diminishing 
1 First by Dr. Shaw in the Naturalist’s Miscellany. 
