The Marsupialia are not entirely confined to Australasia; a few occur in South America, and in North America they are represented by the "'possum," _i.e._ Opossum, of American stories. The Marsupials seem almost to mimic the forms of ordinary quadrupeds. Thus _Notoryctes_, a form discovered a few years ago, mimics a mole. The fact is that, just as among the Eutheria, or higher mammals, special types have become established, possessed of certain habits, and especially of certain habits with regard to food, and modified in accordance with those habits. Thus there are among them savage carnivora, harmless herbivora, and rodents; and these respectively share certain characteristics in common with the carnivora, herbivora, and rodents, belonging to the Eutheria. One of the herbivorous marsupials is the Great Kangaroo, _Macropus_. It gets its name, Large-foot, from the size of its hind-paws; on these it stands, and by their aid it takes remarkably long leaps. Its skull is shown in Fig. 45; this, however, has not the full set of teeth, some of which are soon shed. It crops the herbage with its front teeth, and grinds it with its back teeth, like other herbivora. [Illustration: FIG. 46.--Skull and lower jaw of Rodent; _i_, _i_, incisor teeth, separated by a long interval from the molars. About one-half the natural size.] The study of the teeth is of great help in the classification of the Mammalia. Of the eight orders of the Eutheria, two alone, the Sloth order and the Whale order, show a tendency to the suppression of the teeth. Those of the herbivora and carnivora may easily be compared by anyone, in the sheep and the dog respectively. Fig. 46 shows the skull of a Rodent, with elongated front teeth, adapted for that persistent gnawing which makes the animals of the order, such as the Rat and Rabbit, so terribly destructive. TABLE SHOWING THE CLASSIFICATION OF THE MAMMALIA { 1. PROTOTHERIA, or EGG-LAYING MAMMALS. { One order, the MONOTREMATA. =MAMMALIA.= { { 2. METATHERIA, or MARSUPIAL MAMMALS. { { 3. EUTHERIA, or HIGHER MAMMALS. The Mammalia are a terrestrial group. Exceptions are the Cetacea (Whales), Sirenia (Dugongs), and Seals or Sea-Carnivora, but all of these are air-breathers; even the Whale can only stay under water for a limited period of time. Hence we see that none of them are really animals belonging to the water; they are land animals adapted for life in the water.