Fam. 4. Hystricidae. - In this family are the well-known Porcupines, distinguished from the other Rodents by the fact that the body is covered with long spines or "quills," mixed with bristly hairs. They have four back teeth on each side of each jaw, and they possess imperfect clavicles.

The true Porcupines (Hystrix) have non-prehensile tails, which are mostly furnished with long hollow spines, but sometimes with scales and bristles. As at present restricted, they are found in the Old World only. They are mostly inhabitants of hot climates, with the exception of the common Porcupine (H. cristata), which occurs in Southern Europe and in the north of Africa. In the genus Atherura of Asia and the Indian Archipelago, the tail is long and scaly, and is terminated by a bundle of flattened horny strips.

Fam. 5. Cercolabidae. - This family is hardly separable from the preceding, the chief difference being that the animals composing it spend more or less of their lives in trees, and are therefore adapted for climbing. The Cercolabidae comprise the American Porcupines, of which the principal genera are Erethizon and Cercolabes. In the genus Frethizon, represented by the Canada Porcupine (F. dorsatum) of North America, the quills are short, and are half hidden in the hair, and, though the animal is arboreal in habit, the tail is non-prehensile.

The nearly-allied genus Cercolabes or Sphingurus is South and Central American, and it is distinguished from the preceding by the possession of a long prehensile tail. In fact, the species of Cercolabes, like so many of the inhabitants of this wonderful continent, are adapted for an arboreal life, instead of being confined to the ground.

Fam. 6. Octodontidae. - This family includes a large number of Rodents which are principally South American and African (Octodon, Echimys, Ctenomys, etc.) The best-known species is the beaver-like Coypu (Myopotamus coypus) of South America, in which the hind-feet are webbed, and the tail is long and rounded. It inhabits burrows in the sides of streams, and it leads a semi-aquatic life.

Among the other Octodontidae, the species of Octodon live in South America, and are rat-like Rodents, with short tufted tails, the molars being, typically, of a simple type. In Ctenomys, also South American, the toes of the hind-feet carry laterally a sort of comb of bristles. The Spiny Rats (Echimys) are found in the West Indies and in Africa, and have the hair mixed with fine spines, while the molars have complicated enamel-folds. Petromys is an African type.

Fam. 7. Chinchillidae. - This family includes some South American Rodents, of which the true Chinchillas (Chinchilla) are the best known. They are small, squirrel-like, nocturnal animals, with large ears, and excessively soft fur, strictly terrestrial in their habits, and having the hind-legs considerably longer than the fore-legs. The Alpine Viscachas (Lagidium) live on the Andes up to heights of 16,000 feet; and the Vis-cacha of the plains (Lagostomus) inhabits the South American pampas.

Fam, 8. Castoridae. - The best-known example of this family is the beaver (Castor fiber). The distinctive peculiarities of the family are the presence of distinct clavicles, the possession of five toes to each foot, and the fact that the hind-feet are webbed, adapting the animal to a semi-aquatic life.

The Beaver is a large Rodent, attaining a length of from two and a half to three feet. Naturally it is a social animal, living in societies, and this is still the case in America,* but in northern Europe and Asia, where the animal has been much hunted, it leads a solitary life. When living in social communities the beavers build dams across the rivers, as well as habitations for themselves, by gnawing across the branches of trees or shrubs, and weaving them together, the whole being afterwards plastered with mud. There is no doubt but that the Beaver shows extraordinary ingenuity in these and similar operations; but there can be equally little doubt as to the greatly-exaggerated stories which have been set afloat in this connection. The tail is greatly flattened and scaly, and the animal gives the alarm by striking it upon the water. The Beaver is hunted chiefly for the sake of the skin, but also for the substance known as castoreum. This is a fatty substance, secreted by peculiar glands, and employed as a therapeutic agent.

* The American Beaver is sometimes considered to be a distinct species (Castor Canadensis).

Fam. 9. Saccomydae. - This family comprises the so-called Pouched Rats and Gophers of North America, all of which have large external cheek-pouches. Some of them (Geomys and Thomomys) have the fore-feet greatly developed, and adapted for burrowing; whilst the so-called "Kangaroo-rats" (Dipodomys) have very long hind-legs, and the fore-limbs are not specially developed. The Gophers (Geomys, etc.) possess a pair of cheek-pouches, which are hairy inside, and open outside the mouth, their use being to carry provender. The best-known species is the common Pocket - gopher (Geomys bursarins) of the Mississippi valley and Canada.

Fam. 10. Spalacidae. - Nearly related to the American Gophers are the Mole-rats (Spalax) of the Old World. These have a thick body, short legs, the tail rudimentary or absent, the molars rooted, and the feet five-toed. The Mole-rats are burrowing animals, in which the eyes are very small, and may be covered over by the skin, so as to be functionally useless. They live upon vegetable food, unlike the Mole, and some of them lay up a winter store. Georychus and Bathyergus are African forms of the group.

Fam. 11. Muridae. - The next family of Rodents is that of the Muridae, comprising the Rats, Mice, and Lemmings. In this family the tail is long, always thinly haired, sometimes naked and scaly. The lower incisors are narrow and pointed, and there are complete clavicles. The hind-feet are furnished with five toes, the fore-feet with four, together with a rudimentary pollex. The family comprises over three hundred living species, distributed over the whole world, except the islands of the Pacific, some of the species (such as the Brown Rat and the common Mouse) being similarly cosmopolitan in their range.