A third genus of Flying Phalangers is the minute _Acrobates_, which has a distichous tail like that of _Distaechurus_. It is not more than 6 inches in length including the tail. As to these Flying Phalangers it is exceedingly instructive to observe that the same method of "flight" has been apparently evolved three times; for the three genera are each of them specially related to a separate type of non-flying Phalanger. The same observation can be made about the Flying Squirrels, _Anomalurus_ and _Sciuropterus_. The dental formula is I 3/2 C 1/0 Pm 3/3 M 3/3. The ears are thinly clad with hair. There are four teats. SUB-FAM. 2. PHASCOLARCTINAE.--The Koala, or Native Bear, _Phascolarctos cinereus_, is the only representative of its sub-family. It is, like the Wombat, aberrant in the lack of an obvious tail. The absence of this appendage is curious in an arboreal creature whose near allies have a long and prehensile one. The structure of the Koala was investigated by the late Mr. W. A. Forbes.[89] There are some unexpected points of likeness to the Wombat: thus they agree in the absence of the tail, in the structure of the stomach, and in the great subdivision of the lobes of the liver. The brain, however, is smooth, and the caecum is exceedingly large and complicated in structure, that of the Wombat being short. That both animals have cheek-pouches is perhaps due to similar habits of temporarily storing masses of food. This animal has only eleven pairs of ribs. The tail has only seven or eight vertebra, and these have no chevron-bones. A peculiarity of the skull is seen in the great size of the alisphenoid bulla, which is comparable in size and appearance with that of the Pig. As in the Kangaroos, the atlas is incomplete below. {143} The tooth formula of the genus is I 3/1 C 1/0 Pm 1/1 M 4/(4 or 5). The additional lower molar seems to be exceptional, and has been found in one specimen only. In the alimentary tract the most remarkable structure is the large intestine, which is very capacious for the first 28 inches or so of its course. This section of the colon is lined with rugae precisely like those which are found in the caecum. These folds, which at first are some twelve in number, fuse lower down, and by the time that the colon approaches the external orifice are reduced to five. Similar folds, as already stated, occur in the caecum, but do not extend as far as its blind end. The caecum is proportionately and actually larger than in any other Marsupial. The gall-bladder is unusually elongated. [Illustration] FIG. 70.--Koala. _Phascolarctos cinereus._ x 1/9. The Koala is mainly crepuscular or nocturnal in its habits. It feeds so exclusively upon the leaves of the gum-tree (_Eucalyptus_) that it is impossible to keep the creature long in captivity in lands where that particular kind of food is not available.