The GROUND RAT (_Aulacodus Swinderianus_), of Western and Southern Africa, is remarkable as being the sole representative of this group outside the South American province. It has very broad incisors, and those of the upper jaw exhibit three deep grooves; the molars show the same arrangement of folds as in _Capromys_; the fur is harsh and bristly, and of a general brown tint; and the tail is of moderate length, sparingly haired, dusky above, and whitish below. The fore feet have the thumb rudimentary and the outer toe very short; and the hind feet have only four toes, of which the outer one is rudimentary. This curious animal, which is nearly two feet long, is known to be an inhabitant of Sierra Leone and the Gambia, and also of South Africa (Port Natal); in all probability it occurs at many intermediate localities. In Sierra Leone it is known as the Ground Rat, or Ground Pig, and is said to feed upon ground nuts, and cassada and other roots in search of which it digs into the ground, where it also forms large burrows for its residence. CHAPTER IV. PORCUPINES--CHINCHILLAS--AGOUTIS--CAVIES--HARES AND RABBITS--PIKAS. HYSTRICIDÆ, THE PORCUPINES--Conversion of Hairs into Spines--Skull--Dentition--Tail--Sub-families--The True Porcupines--The Tree Porcupines--THE COMMON PORCUPINE--Distribution--Description--The Crest of Bristles--Nature of the Spines--Habits--Young--Flesh--On the Defensive--Other Species--Species of Tree Porcupines--THE COUENDOU--THE COUIY--Description--Habits--THE URSON, OR CANADA PORCUPINE--Description--Habits--Food--CHINCHILLIDÆ, THE CHINCHILLAS--Characteristics--THE VISCACHA--Description--Life on the Pampas--Their Burrows--Habits--The Chinchillas of the Andes--THE CHINCHILLA--THE SHORT-TAILED CHINCHILLA--CUVIER’S CHINCHILLA--THE PALE-FOOTED CHINCHILLA--DASYPROCTIDÆ, THE AGOUTIS--Characters--THE AGOUTI--Distribution--Appearance--Habits--AZARA’S AGOUTI--THE ACOUCHY--THE PACA--Appearance--Distribution--Habits--DINOMYIDÆ--Founded for a Single Species--Description--Rarity--CAVIIDÆ, THE CAVIES--Characteristics--THE RESTLESS CAVY--Appearance--Habits--The Guinea-Pig Controversy--THE BOLIVIAN CAVY--THE ROCK CAVY--THE SOUTHERN CAVY--THE PATAGONIAN CAVY, OR MARA--Peculiar Features--Its Burrows--Mode of Running--THE CAPYBARA--Its Teeth--Where Found--Habits--THE DOUBLE-TOOTHED RODENTS--Characteristics--LEPORIDÆ, THE HARES AND RABBITS--Structural Peculiarities--Distribution--Disposition--THE COMMON HARE--Hind Legs--Speed--Its “Doubles”--Other Artifices--Its “Form”--Habits--Food--Pet Hares--THE RABBIT--Distribution--Habits--Domesticated--THE MOUNTAIN HARE--LAGOMYIDÆ, THE PIKAS--Characteristics--Distribution--THE ALPINE PIKA--THE ROCKY MOUNTAIN PIKA. FAMILY XII.--HYSTRICIDÆ (PORCUPINES).