The Patagonian Cavy is clothed with a dense crisp fur of a grey colour on the upper part of the head and body, rusty yellow on the flanks, and white on the chin, throat, and belly; the rump is black, with a broad white band crossing it immediately above the tail. It inhabits Patagonia about as far south as 48°, and extends northwards into the La Plata territory as far as Mendoza. It is found only in the sterile desert part of the country, where the gravelly plains are thinly covered with a few stunted thorny bushes and a scanty herbage. The northern limit of the species, according to Mr. Darwin, is at the point where the vegetation of the plains becomes rather more luxuriant. The Patagonian Cavy usually burrows in the ground, but where it lives in the same region as the Viscacha, it will take advantage of the excavations made by that animal. It wanders to considerable distances from its home, and on these excursions two or three are usually seen together. Mr. Darwin says:--“It is a common feature in the landscape of Patagonia to see in the distance two or three of these Cavies hopping one after the other over the gravelly plains.” Their mode of running, on the same authority, more nearly resembles that of the Rabbit than of the Hare; though their limbs are long, they do not run very fast. They rarely squat like a Hare, but are very shy and watchful, and feed by day, in connection with which it is to be observed that the eyes are defended from the direct rays of the sun by well-developed eyelashes, which do not occur in the other Cavies. The female produces generally two young at a birth, which are brought forth and suckled in the burrow. The CAPYBARA (_Hydrochœrus capybara_), the only other member of the present family, is the largest of all existing Rodents, large specimens measuring over four feet in length. It is a stout-built and massive animal, with limbs of moderate length, a large head with a very blunt muzzle, small eyes and ears, no tail, and both the fore and hind feet webbed. The upper incisor teeth have a broad and shallow groove down the front, and the molars present very remarkable characters. In the upper jaw the first three molars are each composed of two lobes united by cement, and on the outside of each of these lobes there is a fold of enamel which passes deeply into the tooth. The last molar consists of one lobe similar to those of the preceding teeth, but in place of the second there are ten or a dozen transverse plates. The first two molars of the lower jaw exhibit complex lobes and folds of enamel; the third and fourth a combination of folded lobes and transverse plates.