CHAPTER VII

THE SUBCLASS EUTHERIA AND THE ORDER EDENTATA

THE whole of the remaining groups of mammals are included in a single subclass, known by the names Eutheria, Monodelphia, or Placentalia.1 The one distinctive feature they have in common (from which the last-mentioned name is derived) is the presence of an allantoic placenta by means of which the foetus is nourished within the uterus of the mother. Throughout the entire subclass, as a general rule, the urino-genital organs open quite independently of the rectum ; the corpus callosum of the brain is well developed; the mandible does not show a marked inflection of its angle; and distinct epipubic bones are not attached to the anterior margin of the pubic symphysis. In those cases where there is a heterodont and diphyodont dentition the dental formula can be reduced to some modification of the one given on p. 25, there being only one known genus where four true molars occur, and even that not invariably. As in the Metatheria, the coracoid is reduced to a mere appendage of the scapula, and the acetabular cavity of the pelvis is imperforate. While the survivors of the other subclasses 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 specialisation in various directions. They consequently exhibit far greater diversity of external or adaptive modification 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 members of this large group, it is usual to separate them into certain

1 The characters of the chief groups of the Eutheria here given are, in some measure, a fuller recapitulation of those already detailed in Chapter III., pp. 83-88.

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 satisfactorily 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 specialised survivors of an ancient group, most of the members of which are extinct, although the researches of palæontology have not yet revealed them to us. The characters of their cerebral, dental, and in many cases of their reproductive organs show an inferior grade of organisation 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 organisation,— 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 generalised mammalian type, show no special characters allying them to any of the other orders. Another equally well-marked and equally isolated, though far more numerously 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.

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 realise. Their dentition especially, however diversified in detail, always responds to the formula already alluded to, 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 group called Proboscidea. These, however, are now known to be the survivors of a large series of similar animals, Mammoths, Mastodons, and Dinotheres, which as we pass backwards in time gradually assume a more ordinary or generalised 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, represented in the actual fauna by the Horses, Rhinoceroses, Tapirs, Swine, and Ruminants. Almost as isolated as the Proboscidea among existing mammals are the few small species constituting the family Hyracidæ, and in their case palæontology 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, though 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 may be included within it. Again, the Rodentia or gnawing animals—Rabbits, Rats, Squirrels, Porcupines, Beavers, etc.-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 Typotherium, 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. It has also been suggested that the Rodents are connected by some of the extinct Tillodontia (or Tæniodontia) with the Edentates. 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 represented in the Eocene deposits both of America and Europe, to which Cope has given the name of Creodonta. The Carnivora also appear to have been closely connected with the primitive Ungulates as represented by the extinct group called Condylarthra. In another direction the step from the Insectivores to the Lemurs is not great, and in past times the transition was probably complete. The Bats or Chiroptera are allied to the Insectivora in all characters except 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 them and all other mammals that, in the absence of any knowledge of any completely intermediate or transitional forms, they can be perfectly separated, and constitute as well-defined an order as any in the class. We have, however, an inkling of the mode in which the Insectivora were modified into Chiroptera shown us by the so-called Flying Lemur (Galeopithecus). Finally, we have the important and well-characterised group called Primates, including all the Monkeys and Man; and the question is not yet solved as to how and through what

forms this is linked on to the other groups. It is commonly assumed that the Lemurs are nothing more than inferior Primates, but the interval between them in the actual fauna of the world is very great, and our knowledge of numerous extinct types recently discovered in America, said to be intermediate in characters, is not yet sufficient to enable us to form a definite opinion upon the subject. The Edentata may be taken first as standing in some respects apart from all the others; and the Primates must be placed at the head of the series. The position of the others is quite arbitrary, as none of the hitherto proposed associations of the orders into larger groups stand the test of critical investigation, and palæontological researches have already gone far to show that they are all modifications of a common heterodont, diphyodont, pentadactylate form.

Order EDENTATA.

The name assigned to this group (which some zoologists think ought rather to be ranked as a subclass 1 than an order) by Cuvier is often objected to as inappropriate for although some of the members are edentulous, others have very numerous teeth-and the Linnæan name Bruta is occasionally substituted. But that term is quite as objectionable, especially since the group to which Linnæus applied it is by no means equivalent to the order as now understood, as the names of the genera contained in it, viz. Elephas, Trichechus, Bradypus, Myrmecophaga, Manis and Dasypus, indicate. It contained, in fact, all the animals then known which are comprised in the modern groups of Proboscidea, Sirenia and Edentata together with the Walrus, one of the Carnivora. If retained at all, it should rather belong to the Proboscidea, as Elephas stands first in the list of genera in the Systema Natura. Cuvier's order included the Ornithorhynchus and Echidna, the structure of which was then imperfectly known, and which are now by common consent removed to an altogether different section of the class; but otherwise its limits are those now adopted. The name Edentata is so generally used, and its meaning so well understood, that it would be undesirable to change it now; in fact similar reasons might be assigned for ceasing to use nearly all the other current ordinal designations, for it might be equally well objected that all Carnivora are not flesheaters, many of the Marsupialia have not pouches, and so forth.

If the teeth are not always absent, they invariably exhibit certain imperfections, which are indeed almost the only common characters binding together the various extinct and existing members of the order. These are that they are homodont and, with the

1

1 The name Paratheria has been suggested for this proposed subclass.

remarkable exceptions of Tatusia and Orycteropus, monophyodont; they are never rooted, but have persistent pulps; except in some fossil forms, they are always deficient in one of the constituents which enter into the formation of the complete mammalian tooth, the enamel; and, at least among living forms, are never present either in the upper or lower jaw in the fore part of the mouth, the situation occupied by the incisors of other mammals.1

The peculiar nature of the dentition in the aberrant Orycteropus will be noticed under the heading of that genus. As a rule, the coracoid process of the scapula of the Edentates is more developed than in other Eutheria.

The degree of development of the brain varies considerably in the different families, the hemispheres being in some cases almost or quite smooth (Fig. 57), with a small corpus callosum, and large anterior commissure; while in other instances the hemispheres are convoluted, and the corpus callosum is larger.

There is so great a difference in structure and habits between some of the existing animals assigned to this order that, beyond the negative olfactory lobes are seen at the anterior extremity characters just mentioned, (left of figure); the hemispheres have only three there seems little to connect sulci. (From Garrod, Proc. Zool. Soc. 1878, p. 230.) them. The Sloths and Anteaters, for instance, in mode of life, general conformation of limbs, structure of digestive organs, etc., appear at first sight almost as widely separated as any mammals. Palæontology has, however, thrown great light upon their relations, and proved their real affinities. Perfectly intermediate forms have been discovered in the great Ground Sloths of America, which have the dentition and general form of the head of the Sloths, combined with the limbs and trunk of the Anteaters. It is, indeed, highly probable that the existing members of this order are very much differentiated representatives of a large group, the greater number of which are now extinct, and have become so without ever attaining a high grade of organisation. The great diversity of structure in the existing families, the high degree of specialisation to which many have attained, the paucity of species and even of individuals, their

FIG. 57.--Upper surface of the brain of the Broadbanded Armadillo (Xenurus unicinctus). The large

In some few Armadillos the suture between the premaxilla and maxilla passes behind the first upper tooth; but in all other known members of the order all the teeth are implanted in the maxilla.