The Marsupial animals included in this family are not found in Australia or in Van Diemen’s Land, or in any part of the natural history province to which those countries belong. They are numerous, however, and are now living on the American continent; but formerly some inhabited Europe during that geological period which is called the Eocene. The Opossums are very rat-like in form, the largest species being about the size of a large Cat, but they have the snout more elongated; and in some species in which the individuals are large the body is proportionately stout, and on most there is a comfortable fur, with short and long hair. The tail is almost always very long, nearly destitute of hair, excepting at the root, and is covered with a scaly skin, there being a few scattered hairs. It is a useful organ, for the Opossums hang by it, and it assists them in climbing and descending trees, and in holding on, when they are young, to their parent. The ears are rather large and round, the eyes are placed rather high up in the face, and the long muzzle ends in a naked snout. The legs look short for the body. The feet are naked beneath; there are five toes, and the great toe is more or less opposable to the foot, and acts like a grasping thumb. Each toe is furnished with moderate-sized claws, excepting the inner toe of the hind foot, which is clawless. The Opossums are remarkable for the great number of their incisor teeth, there being ten in the upper and eight in the lower jaw, and they are arranged in a semicircular manner. The upper and two foremost incisors are rather longer than the rest, and are generally separated from them by a narrow space. They are nearly cylindrical and expanded at the tip. The canines are well developed, the upper ones being the largest. There are three premolars on each side of both jaws, and they have two roots, and are compressed and pointed. There is a posterior talon to them. The molars, eight in each jaw, have three roots, and those of the upper jaw have the crown of a triangular form and tubercular, whilst those of the lower jaw are longer than broad, and each has the appearance of five prickly cusps on its upper surface. [Illustration: TEETH OF THE OPOSSUM.] Some of the Didelphidæ have no marsupium, or pouch, or it is very slightly developed, and in these particular kinds the young, after having left the nipples, are carried on the back of the mother, retaining their position by twining their tails around hers. The mammæ are numerous: there may be as many as thirteen, an odd one being found in the centre of the ring of the other nipples.