This family, the last of the simple-toothed Rodents, includes a small number of species, of which the Common Guinea-pig may serve as a sort of type. The Guinea-pig is, however, one of the smaller species of the family, and is shorter in the limbs than most of its relatives. They have the incisor teeth short, that is to say, not extending far back in the jaw; the molars are rootless, variously divided by folds of enamel into lobes, the angles of which are acute; the palate is narrow in front, so that the upper series of molars approach each other rather closely in front; the clavicles are rudimentary or wanting; the fore limbs have four and the hind feet only three toes, all armed with hoof-like nails; the upper lip is not cleft; and the tail is rudimentary or wanting. They are stout, more or less rabbit-like animals, with a soft coat, and the ears variable in length; and they are confined to the continent of South America, where they chiefly inhabit the plains. The RESTLESS CAVY (_Cavia aperea_), which is commonly regarded as the wild original of the so-called Guinea-pig (_Cavia cobaya_ of some authors), is abundant on the banks of the Rio de la Plata, and extends thence northwards through Paraguay into Bolivia and Brazil. It is usually about nine inches long, with the fur of the upper part and sides of the body composed of a mixture of black and dingy yellow hairs, the chest greyish-brown, and the throat and belly pale dingy-yellow or brownish-grey. The incisor teeth are white. The genus to which this animal belongs may be at once distinguished from the other two genera constituting the family by the shortness of the limbs; the ears also are short; the feet are naked beneath; the hind toes are not webbed; and the molar teeth are nearly equal in size, and each composed of two angular lobes.