This family has been founded for the reception of a single species, of which only a single specimen has hitherto been obtained. It is described by Professor Peters under the name of _Dinomys Branickii_. In its external appearance it closely resembles the Paca (_Cœlogenys paca_), but may at once be distinguished from that animal by its possession of only four toes both before and behind. The ears are short and rounded; the upper lip deeply cleft; the incisors very broad; the molars four on each side, and divided into transverse plates by folds of enamel; the clavicles are imperfect; and the tail of moderate length and well clothed with hair. The animal, which inhabits the high mountain regions of Peru, is of the size of the Paca, or about two feet long, exclusive of the tail, which measures rather more than nine inches. Its general colour is grey, produced by the sprinkling of white among nearly black hairs; and on each side of the body are numerous large white spots, of which the upper ones nearly run together, so as to form two longitudinal bands. The extremity of the tail is black. The only known example of this Rodent was obtained by M. Constantin Jelski at the Colonie Amable Maria, on the Montaña de Vitoc, in Peru, having been found at daybreak walking about the yard. It showed no fear of man, and was easily killed by a sword cut or two on the head. The species would appear to be rare, as the inhabitants of the neighbourhood were not acquainted with it. Of course nothing is known of its habits. [Illustration: THE DINOMYS. (_After Peters._)] The chief interest at present attaching to this animal, therefore, consists in its peculiar combination of characters. Externally, as already stated, it resembles the Paca, with which it also agrees in the S-like form of the nostrils, and in the structure of the limbs (except the number of toes). In the lamellar structure of the molar teeth, in the structure of the skull, and of the skeleton generally, and especially in the flattened form of the front of the sternum and the development of clavicles, it differs from the Paca and all other Rodents with hoof-like nails. In some minor particulars it resembles the Capybara. By the structure of the molar teeth and certain osteological characters, it is most nearly allied to the Chinchillas; while it approaches the genus _Capromys_ among the Octodontidæ in the structure of the limbs and of some other parts of the skeleton. Professor Peters is evidently inclined to regard it as most nearly related to the Chinchillidæ, but as constituting a group establishing a closer union than previously existed between the families Chinchillidæ, Octodontidæ, Dasyproctidæ, and Caviidæ. FAMILY XVI.--CAVIIDÆ (CAVIES).