The Rat and Mouse form the types of a great sub-family, MURINÆ, which have the molars rooted and tuberculate when young, the infra-orbital opening high and perpendicular, widest above, and the lower root of the zygomatic maxillary process flattened into a perpendicular plate. They possess no cheek-pouches, have the fore and hind limbs approximately equal in length, the thumb rudimentary, and the tail nearly naked, covered with scaly rings. The genus _Mus_, to which our household pests belong, includes upwards of one hundred species, scattered over most parts of the Eastern Hemisphere, and living sometimes chiefly in the neighbourhood of human habitations, granaries, &c., where they often feed indifferently upon animal and vegetable substances, sometimes in the open country, and feeding almost exclusively upon the latter. The common BROWN RAT (_Mus decumanus_), sometimes called the Norway Rat, which is almost too well known to need description, is not a native of Great Britain, but was certainly introduced there by commerce, probably from some southern or eastern country--perhaps, as Pennant thinks, from the East Indies. Haunting ships in great numbers, it has now been introduced into all parts of the world, and it is quite impossible to ascertain its original habitat. It was known in Asia long before it made its appearance in Europe; and its passage into Russia is fixed by Pallas in the year 1727, when, he says, after an earthquake it swam across the Volga from the countries bordering the Caspian. Its first appearance in France and England is said to have occurred about the middle of the last century. [Illustration: BLACK RAT.] From its great fecundity and determined ferocity of disposition, the Brown Rat has become a great pest wherever it has taken up its abode. “It digs,” says Professor Bell, “with great facility and vigour, making its way with rapidity beneath the floors of our houses, between the stones and bricks of walls, and often excavating the foundations of dwellings to a dangerous extent. There are many instances of their fatally undermining the most solid mason-work, or burrowing through dams which had for ages served to confine the waters of rivers and canals.” It is almost impossible to keep them out of our houses, and, once in, there is no end to the mischief they do. Their ferocity is very great; and although they will, if possible, retreat from a powerful enemy, they will fight in the most savage fashion when they cannot escape.