372 MAMMALIA [CLASSIFICATION. uterine and Fallopian portions, and open into a long and distinct vagina, quite separate from the cystic urethra. The penis is large, but its crura are not directly attached to the ischia. The spongy body has a large bifurcated bulb. The young are born in an exceedingly rudimentary condition, and are never nourished by means of an allantoic placenta, but are transferred to the nipple of the mother, to which they remain firmly attached for a considerable time, nourished by the milk injected into the mouth by compression of the muscle covering the mammary gland. They are therefore, as previously remarked (see p. 369), the most typically mammalian of the whole class. The nipples are nearly always concealed in a fold of the abdominal integument or " pouch " (marsupium) which serves to support and protect the young in their early helpless condition. The existing species of this group are entirely confined to the Australian region and the American continent, though in former times they had a more ex tended geographical range. The earliest mammals hitherto discovered appear (as far as the scanty evidence at present obtainable permits any such conclusion to be hazarded) to have belonged to this type, although it is reasonable to conclude that Prototheria (unless upon the improbable supposition that the existing forms have resulted from a process of degradation), and perhaps Eutheria, were their contemporaries far back in the Mesozoic age Euthevia. The Eutheria, Monoddplda, or " placental mammals " (so called because the foetus is always nourished while within the uterus of the mother by means of an allantoic placenta) include at present by far the greater proportion of the class. While the survivors of the other groups have probably been for a long time in a stationary condition, these have, as there is already good evidence to show throughout all the Tertiary geological age, and by inference for some time before, been multiplying in numbers and variations of form, and attaining higher stages of development and specialization in various directions. They consequently exhibit far greater diversity of external or adaptive modi fication than is met with in either of the other subclasses, some being fitted to live as exclusively in the water as fishes, and others to emulate the aerial flight of birds. To facilitate the study of the different component mem bers of this large group, it is usual to separate them into certain divisions which are called " orders." In the main zoologists are now of accord as to the general number and limits of these divisions among the existing forms, but the affinities and relationships of the orders to one another are far from being understood, and there are very many extinct forms already discovered which do not fit at all satis factorily into any of the orders as commonly defined. Commencing with the most easily-distinguished, we may first separate a group called Edentata, composed of several very distinct forms, the Sloths, Anteaters, and Armadillos, which under great modifications of characters of limbs and digestive organs, as well as habits of life, have just enough in common to make it probable that they are the very specialized survivors of an ancient group, most of the members of which are extinct, but which the researches of palaeontology have not yet revealed to us. The characters of their cerebral, dental, and in many cases of their repro ductive organs show an inferior grade of organization to that of the generality of the subclass. The next order, about the limits of which there is no difficulty, is the Sirenia, aquatic vegetable-eating animals, with complete absence of hind limbs, and low cerebral organization, represented in our present state of knowledge by but two existing genera, the Dugongs and Manatees, and by a few extinct forms, which, though approaching a more generalized mammalian ty,pe, show no special characters allying them to any of the other orders. Another equally well-marked and equally isolated, though far more numer ously represented and diversified order, is that of the Cetacea, composed of the various forms of Whales, Dolphins, and Porpoises. In aquatic habits, external fish-like form, and absence of hind limbs they resemble the last, though in all other characters they are as widely removed as are any two orders among the Eutheria. The association by systematists of the Cetacea and Sirenia in one group can only be made either in ignorance of their true structure, or in an avowedly artificial system. All the remaining orders are more nearly allied together, the steps by which they have become modified from one general type being in most cases not difficult to realize. Their dentition especially, however diversified in detail, always responds to the formula already described (see p. 353) ; and, although the existing forms are broken up into groups in most cases easy of definition, the discoveries already made in paleontology have in great measure filled up the gaps between them. Very isolated among existing Eutheria are the two species of Elephant constituting the order called Probos- cidea. These, however, are now known to be the survivors of a large series of similar animals, Mammoths, Mastodons, and Dinotkeria, which as we pass backwards in time gradu ally assume a more ordinary or generalized type ; and the interval which was lately supposed to exist between even these and the rest of the class is partially bridged over by the discovery in American Eocene and early Miocene formations of the gigantic Dinocerata, evidently offshoots of the great group of hoofed animals, or Ungulata, repre sented in the actual fauna by the Horses, Rhinoceroses, Tapirs, Swine, and Ruminants. Almost as isolated as the froboscidea among existing mammals are the few small species constituting the genus Hyrax, and in their case palaeontology affords no help at present, and therefore, pending further discoveries, it has been thought advisable in most recent systems to give them the honour of an order to themselves, under the name of Hyracoidea. But the number of extinct forms already known allied to the Ungulata, but not coming under the definition of either of the two groups (Artiodactyla and Perissodactyla) under which all existing species range themselves, is so great that either many new orders must be made for their reception or the definition of the old order Ungulata so far extended as to receive them all, in which case both Proboscidea and Hyracoidea might be included within it. Again the Rodentia, or gnawing animals Rabbits, Rats, Squirrels, Porcupines, Beavers, &c. are, if we look only at the present state of the class, most isolated. No one can doubt what is meant by a Rodent animal, or have any difficulty about defining it clearly, at least by its dental characters ; yet our definitions break down before the extinct South American Megatherium, half Rodent and half Ungulate, which leads by an easy transition to the still more truly Ungulate Toxodon, for the reception of which a distinct order (Toxodontia) has been proposed. The Insectivora and the Carnivora again are at present quite distinct orders, but they merge into one another through fossil forms, and are especially connected by the large group of primitive Carnivora, so abundantly repre sented in the Eocene deposits both of America and Europe, to which Cope has given the name of Creodonta. The transition from the Insectivores to the Lemurs is not great and, strange to say, however different they nosv appear, the early forms of Lemurs are not easily distinguished from the primitive Ungulates. The Bats or Chiroptera are allied to the Insectivora in all characters but the extraordinary modification of their anterior extremities into wings, but this, like the want of the hind limbs in the
Cetacea and Sirenia, makes such a clear distinction between