The lowest order of the placental or monodelphous Mammals is that of the Edentata, often known by the name of Bruta. The name Edentata is certainly not an altogether appropriate one, since it is only in two genera in the order that there are absolutely no teeth. The remaining members of the order have teeth, but these are always destitute of true enamel, are never displaced by a second set, and have no complete roots. Further, in none of the Edentata are there any median incisors, and in only one species (one of the Armadillos) are there any incisor teeth at all. Canine teeth, too, are almost invariably wanting. Clavicles are usually present, but are absent in the Scaly Ant-eater (Manis). All the toes are furnished with long and powerful claws. The mammary glands are usually pectoral, but are sometimes abdominal in position. The testes are abdominal in position. The skin is often covered with bony plates or horny scales.
The placentation of the Edentates varies, the placenta being discoidal and deciduate in the Sloths (e.g., Choloepus Hoffmanni), but diffuse and non-deciduate in Manis (Turner) - a fact which throws some doubt on the propriety of using the placental characters in classification.
The order Edentata is conveniently divided into two great sections, in accordance with the nature of the food, the one section being phytophagous, the other insectivorous. In the former section is the single group of the Sloths (Bradypodidae). In the latter are the two groups of the Armadillos (Dasypodidae), and the various species of Ant-eaters (the latter constituting Owen's group of the Edentula).
The order Edentata is but sparingly represented in modern times, and its geographical distribution is peculiar. The true Ant-eaters, the Armadillos, and the Sloths, are entirely confined to South America, in which country a group of gigantic extinct Edentates existed in Post-tertiary times. The Scaly Ant-eater or Manis is common to Asia and Africa, and the genus Oryc-teropus is peculiar to Africa.
The family Bradypodidae (or Tardigrada) comprises some exceedingly curious animals, which are exclusively confined to South America, inhabiting the vast primeval forests of that continent. The Sloths have a remarkably short and rounded face, and the body is covered with hair. The mammae are two in number and pectoral in position; and the tail is short or quite rudimentary. The incisor teeth are altogether wanting (fig. 372, A), but there is always a small number of simple molars, and in the Two-toed Sloths or Unaus the first tooth in each jaw on each side is so much larger than the others, and so much more pointed, that it has been regarded as a canine. The malar bone is not directly articulated with the temporal bone, and it sends backwards two long processes, directed respectively upwards and downwards (fig. 372, A). The stomach is complex, somewhat resembling that of the Ruminants. The cervical vertebrae are more than the normal seven in number in the Three-toed Sloth, and less than the normal in one of the two-toed species; and the long bones have no medullary cavities.
Fig. 372. - A, Side-view of the skull of Bradypus cuculliger; B, Side-view of the skull of Dasypus gigas. (After Giebel.)
The most striking peculiarities, however, about the Sloths are connected with their mode of life. The Sloths, in fact, are constructed to pass their life suspended from the under surface of the branches of the trees amongst which they live; and for this end their organisation is singularly adapted. The fore-limbs are much longer than the hind-limbs, and the bones of the fore-arm are unusually movable. All the feet, but especially the fore-feet, are furnished with enormously long curved claws (fig. 373), by the aid of which the animal is enabled to move about freely, suspended back downwards from the branches. Not only is this the ordinary mode of progression among the Sloths, but even in sleep the animal retains this apparently unnatural position.
Fig. 373. - Hand of Three-toed Sloth (Bradypus tridactylus). (After Owen.)
Owing to the disproportionate size of the fore-limbs, as compared with the hind-limbs, and owing to the fact that the hind-feet are so curved as to render it impossible to apply the sole to the ground, the Sloth is an extremely awkward animal upon the ground, and it has therefore recourse to terrestrial progression only when absolutely compelled to do so. Whilst the name of "Sloth" may thus appear to be a merited one from the point of view of a terrestrial Mammal, it is wholly undeserved when the animal is looked upon as especially adapted for an arboreal existence. In the Ai or Three-toed Sloth (Bradypus tridactylus) there are three toes to each foot, and these are short, completely rigid, and so enveloped in the integument as to leave nothing visible except the enormously long and crooked claws. The hand and foot are jointed to the arm and leg obliquely, so that the palm and sole cannot be applied to the ground, but are turned inwards. The ungual phalanges are also so articulated that the claws are bent inwards towards the palm or sole. There are sixteen pairs of ribs. The molars are rootless, growing from permanent pulps, and consisting of a simple cylinder of dentine enveloped in cement. In the Unau (Choloepus) the forefeet are two-toed, and there are twenty-three pairs of ribs, the greatest number known in the Mammals.
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The second family of the Edentata is that of the Dasypodidae or Armadillos. These are found exclusively in South America, as are the Sloths, but they are very different in their habits. The Armadillos are burrowing animals, furnished with strong digging-claws and well-developed collar-bones. They feed upon insects, worms, carrion, roots, and fruits. The jaws are provided with numerous simple molars (fig. 372, B), which attain the enormous number of nearly one hundred in the great Armadillo (Priodontes gigas). The upper surface of the body is covered with a coat of mail, formed of hard bony plates or shields united at their edges (fig. 374). A portion of this armour covers the head and shoulders, and another portion protects the hind-quarters; whilst between these is generally a variable number of movable bands which run transversely across the body and give the necessary flexibility to this singular dermoskeleton. In most species this flexibility is so great that the animal can roll itself up like a hedgehog. The tail is likewise mostly covered with bony scutes. The spinous processes of the second cervical and of all the dorsal vertebrae are specially developed to carry the dermal shield. The sternum and first rib are expanded, and sternal ribs are present.
Fig. 374. - The three-banded Armadillo (Tolypeutes conurus), one-third of the natural size. (After Murie.)
The Armadillos are confined entirely to America, ranging from Mexico to Patagonia. In this country, also, have been found the remains of the gigantic armour-plated animals allied to the Armadillos, which will be subsequently described under the name of Glyptodon. Amongst the best-known species of Armadillo are the Peba (Dasypus Peba), the Poyou (D. sex-cinctus), the Tatouay (D. Tatouay), the Pichiy (D. minutus), the Peludo (D. villosus), and the Great Armadillo (Priodontes gigas). A somewhat aberrant form is the Chlamyphorus, or Chlamydophorus, of Chili, the total length of which is only about six inches.
The remaining members of the Edentata are the various Ant-eaters; but these are so different from one another in their characters that they form three distinct families, also distinguished by their geographical distribution.