In the lower jaw similar varieties occur in these small and unimportant teeth; e. g. there may be between the procumbent incisors and the posterior premolar, either three teeth as in Phal. Cookii and Phal. cavifi•ons, or two, as in Phal. ursine, Phal. maculate, Phal. chry sorrhoos ; or finally one, as in Phal. vulpine and Phut. faliginosa. The most important modification is presented by the little Phal. gliriforniis of Bell, which has only three true molars on each side of each jaw. As these modifications of the teeth are unaccompanied by any change of general structure or of habit, whilst those teeth which most influence the diet are constant, it is obvious that these differences of dentition are unimportant, and afford no just grounds for subgeneric distinctions.
The Phalangers, being provided with hinder hands and prehensile tails, are strictly arboreal animals, and have a close external resemblance to the Opossums, by which name they are generally known in Australia and the Islands of the Indian Archipelago, where alone they have hitherto been found. They differ from the Opossums chiefly in their dentition; and in accordance with this difference their diet is more decidedly of a vegetable kind.* The Australian Phalangers feed chiefly on the ten der buds and the leaves of Eucalypti : but according to Temminck,f the Indian Phalan gers are omnivorous, and combine insects with fruits and leaves. Mr. Ogilby: states that both " the Phalangers and Petaurists display so decided a preference for live birds, as to make it probable that these constitute a main portion of their food in a state of nature." I find, however, that the intestinal canal, and especially the ccecum, offers so great an addi tional development in length, as, with the cor responding predominance of the incisors, and atrophy of the canines, to indicate clearly a natural and constant tendency in the Phalangers to a vegetable diet. Guided thernfore by the totality of their organization, I am led to place them in a distinct section from that which con tains the Opossums, but, in that section, they come the nearest to the true Opossums. The Phalangers of the Indian Isles have short ears and the greater part of the tail naked. To this group have been applied the names Ceonyx, Cuscus, and Bulantia ; the Australasian Pha langers have moderately long ears, and the greater part, or else the whole of the tail is covered with hair. All the species possess considerable freedom of lateral movement in the anterior digits, and in some small species, as Phal. glirifiirmis, Bell, they appear to be naturally divided into two groups, the two outer being opposed to the three inner fingers. To the hairy-tailed Phalangers exhibiting this character, Mr. Ogilby gives the subgeneric name Pseudocheirus, restricting the term Phalan gista to the remaining species. With reference to the subgenera Cuscus, Balantia, Pseudo cheirus, &c. I heartily concur in the opinion of the experienced and judicious Temminck,* that these numerous sections are perfectly use less, and a burthensome charge .to the memory.
Genus PETAURUS.
There are many species of Marsupialia limi ted to Australia and closely resembling, or identical with, the true Phalangers in their den tal characters and the structure of the feet. I allude to the Petaurists or Flying Opossums : these, however, present an external character so easily recognizable, and influencing so ma terially the locomotive faculties, as to claim for it more consideration than the modifica tions of the digits or spurious molars which we have just been considering in the Pha langers. A fold of the skin is extended on each side of the body between the fore and hind legs, which, when outstretched, forms a lateral wing or parachute; but which, when the legs are in the position for ordinary support or progression, is drawn close to the side of the animal by the elasticity of the subcutaneous cellular membrane, and there forms a mere te gumentary ridge. These delicate and beautiful Marsupials have been separated generically from the Phalangers under the name of Petaurus: they further differ from the Phalangers in want ing 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 con stancy in the exact formula of the dentition as among the Phalangers. The largest species of Petaurus ( Pet. Taguenoides) for example, is almost identical in this respect with the Pha langista Cookii, which M. Fr. Cuvier has therefore classed with the Petauri. Those teeth of Pet. Taguanoides which are sufficiently developed, and so equal in length, as to exer cise the function of grinders, or in other words, the functional series of molars, includes 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 Ph. Cookii, has only three cusps. In the upper jaw the space between the functional false molars and the in cisors is occupied by two simple rudimentary teeth, the anterior representing the canine ; but being relatively smaller than in Ph. Cookii, the crowns of the two anterior incisors are rela tively larger. In the lower jaw the sloping alveolar surface between the functional molars and large procumbent incisors is occupied, ac cording to 111. Fr. Cuvier, by two rudimentary minute teeth, as represented in the figure (fig. 88). I have not found any trace of these in the two skulls of Pet. Taguanoides examined by me. In Ph. Cookii we have seen that there are three minute teeth in the correspond ing space ; but these differences would not be sufficient ground to separate generically the two species if they were unaccompanied by modifications of other parts of the body.