The Rat-tailed Kangaroo-Rat is about fifteen inches and a half long, and the tail measures, in addition, more than nine inches. It has a long head and rather short hind feet, and the rat-like tail has short stiff hairs on it which do not quite hide the scaly skin beneath. The body fur is long and loose, and dusky brown, more or less tinted with black and pale yellowish-brown. The end of the nose or muzzle is spotted, and the ears are short and rounded. This little animal lives in New South Wales, and was that which was first described by Hunter under the name of Potoroo, or Poto Roo, being the “Bettong” of the natives of New South Wales. The stomach of the Kangaroo-Rats is less sacculated than that of the Kangaroos, but its left-hand portion is enormously developed in proportion to the rest, and may be compared with that of the Ruminantia in point of relative size. It may be noticed that the lower jaws of the Potoroos, which are largely inflected at the angle, articulate with the skull rather differently to those of the Kangaroos. In these last, the cavity at the base of the zygomatic process which receives the lower jaw is broad and slightly convex, permitting considerable side-to-side movement which is useful in the occasional “cud chewing.” But in the others the cavity barely deserves the name, it being a nearly flat surface, and, therefore, not much motion, except that of an up-and-down kind, is possible to the jaw. The organ of hearing has been slightly noticed in the Great Kangaroo in a former page, and it is necessary to observe that the tympanic bone does not form a perfect tube in the Potoroos as in the Kangaroos, and that the surface of the auditory cavity is also increased by a “bulla,” or bony cavity, bulging out at the under part of the skull. Corresponding “bullæ” were noticed in the Rodentia, but in their case the swelling is in the temporal bone, whilst in the Marsupials, with the exception of the Wombat, they are formed out of the sphenoid bone (the great ala). Moreover, the Potoroos, like the Kangaroos, and some of the other Marsupials (the Phalangers and Koalas), have the ear chamber prolonged, by a number of cells, into the zygomatic process of the temporal bone. The Kangaroo-Rats are numerous, and there are many species. They are distributed in New South Wales, Western Australia, Van Diemen’s Land, and South Australia, and to the north-east. [Illustration: FORE (A) AND HIND (B) FOOT OF HYPSIPRYMNUS.]