In the members of the typical genus _Potorous_ (formerly known as _Hypsiprymnus_) the head is long and slender, with the auditory bullae somewhat swollen; while the ridges on the first two premolars are few and perpendicular, and there are large vacuities on the palate. The tarsus is short and the muzzle naked. The genus includes _P. tridactylus_, _P. gilberti_ and _P. platyops_. In _Bettongia_, on the other hand, the head is shorter and wider, with smaller and more rounded ears, and more swollen auditory bullae. The ridges on the first two premolars are also more numerous and somewhat oblique (fig. 4); the tarsus is long and the tail is prehensile. The species include _B. lesueuiri_, _B. gaimardi_ and _B. cuniculus_. The South Australian _Caloprymnus campestris_ represents a genus near akin to the last, but with the edge of the hairy border of the bare muzzle less emarginate in the middle line, still more swollen auditory bullae, very large and posterially expanded nasals and longer vacuities on the palate. The list is completed by _Aepyprymnus rufescens_, which differs from all the others by the hairy muzzle, and the absence of inflation in the auditory bullae and of vacuities in the palate. Perhaps, however, the most interesting member of the whole group is the tiny musk-kangaroo (_Hypsiprymnodon moschatus_) of north-east Australia, which alone represents the sub-family _Hypsiprymnodontinae_, characterized by the presence of an opposable first toe on the hind-foot and the outward inclination of the penultimate upper premolar, as well by the small and feeble claws. In all these features the musk-kangaroo connects the _Macropodidae_ with the _Phalangeridae_. The other teeth are like those of the rat-kangaroos. (W. H. F.; R. L.*)