There are about ten species of this genus known, of which the commonest is Cook’s Ring-tailed Phalanger (_Pseudochirus peregrinus_), an animal discovered by Captain Cook during his first voyage, at Endeavour river, North Queensland. The complex and sub-selenodont character of the molars of this and the following genus readily distinguish them from the more typical Phalangers, and show an approximation to the type of dentition prevailing in _Phascolarctus_; according, however, to Mr. O. Thomas, a tendency towards the same structure is observable in unworn molars of young Cuscuses. The genus may be divided into three groups, of which the first, as typified by the common _P. peregrinus_, is restricted to Australia and Tasmania, while the third, as represented by _P. canescens_, is only found in New Guinea. _P. albertisi_ may be taken as the type of the second group, which is represented by that species in New Guinea, and by _P. archeri_ in Queensland. With the exception of _P. peregrinus_, the species have a more or less restricted range. Remains of _Pseudochirus_, probably referable to existing species, are found in the cave-deposits of New South Wales. _Petauroides._[58]—With the genus _Petauroides_, containing only the single species _P. volans_, we come to the first of the Flying-Phalangers, characterised by the possession of a living membrane along the flanks. The characters of this genus are as follows. Size large. Fur very long and silky. Ears large and oval, thickly furred on the back, but naked internally. Flying-membrane reaching from wrist to ankle, but very narrow along the sides of the forearm and lower leg. Fore toes subequal, their relative lengths in the order 4, 3, 5, 2, 1. Claws long, curved, and sharp. Tail long, cylindrical, and bushy, except near its tip, where it is naked and prehensile. Skull short and broad, with the nasals short, and not extending nearly as far forwards as the premaxillæ. Large vacuities in hinder part of palate. Auditory bullæ inflated and smooth. Dentition usually _i_ ³⁄₂, _c_ ¹⁄₀, _p_ ²⁄₂, _m_ ⁴⁄₄. General characters of teeth very similar to those of _Pseudochirus_, but the first upper incisor scarcely longer than the second. The single species is found in Australia, from Queensland to Victoria, and is commonly known as the Taguan Flying-Phalanger. The structure of the skull and teeth indicates close affinity with _Pseudochirus_, although the external form is widely different in the two genera. This Phalanger seems, indeed, to be, so to speak, a very specialised _Pseudochirus_, in which the teeth have become somewhat further diminished and the flying membrane has been developed.