Page:Encyclopædia Britannica, Ninth Edition, v. 15.djvu/406

Rh 384 MAMMALIA [EDENTATA. reasons might be assigned for ceasing to use nearly all the other current ordinal designations, for it might be equally well objected that all the 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 which bind together the various extinct and existing members of the order. These are that they are homodont and, with the remarkable exception of the genus Tatusia, monophyodont ; they are never rooted but have persistent pulps ; they are always deficient in one of the constituents which enter into the formation of the complete mammalian tooth, the enamel, and 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 There is so great a difference in structure and habits between some of the existing animals assigned to this order that, beyond the negative characters just mentioned, there seems little to connect them. The Sloths and Anteaters, for instance, in mode of life, general conforma tion of limbs, structure of digestive organs, &c., appear at first sight almost as widely separated as any mammals. Palaeontology has, however, thrown great light upon their relations, and proved their real affinities. Perfectly inter mediate 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 highly probable that the existing members of the order are very much differen tiated representatives of a large group, the greater number of which are now extinct, and which have become so with out ever attaining a high grade of organization. The great diversity of structure of the existing families, the high degree of specialization to which many have attained, the paucity of species and even of individuals, their limited area of distribution, and their small size compared with known ancestral forms, all show that this is an ancient and a waning group, the members of which seem still to hold their own either by the remoteness and seclusion of their dwelling-places, by their remarkable adaptation of structure to special conditions of life, or by aid of the peculiar defensive armature with which they are invested. Their former history can, however, only be thus surmised, rather than read, at present ; for, though we have ample evidence of the abundance and superior magnitude of certain forms in the most recent or post-Tertiary geological age, and in one part of the world, beyond this time, i.e., in the true Ter tiary period, and in other parts of the world than America, the remains of animals of this order hitherto discovered are only fragmentary, giving a most imperfect idea of their actual structure, and affording no indications which serve to connect them with any other branch of the class. The existing members of the order readily group them selves into five distinct families, the limits of which are perfectly clear. These are (1) Bradypodidx, or Sloths; (2) Myrmecophagidaz, or Anteaters ; (3) Dasypodidx, or Armadillos ; (4) Manidse, Pangolins or Scaly Anteaters ; and (5) Orycteropodidse,, Aard-varks or African Anteaters. The geographical distribution of these families coincides with their structural distinction, the first three being inhabitants of the New and the last two of the Old World. It has been usual to arrange these families into two large groups or suborders : (1) the Phyllophaga, leaf-eaters, also called Tardigrada, containing the Bradypodidse alone ; and (2) the Entomophaga, insect-eaters, or Vermilingua, contain ing all the other families, from which sometimes the Orycteropodidx are separated as a third suborder under the 1 In some few Armadillos the suture between the prem axilla and maxilla passes behind the first upper tooth, but in all the other known members of the order all the teeth are implanted in the maxilla. name of Effodientia. Such an arrangement is, however, an artificial one, founded on superficial resemblance. The bonds which unite the Manidx to the Myrmecophagidx are mainly to be found in the structure of the mouth, especially the extensile character of the tongue, the great develop ment of the submaxillary glands, and the absence of teeth. These characters are exactly analogous to those found in the Echidna among Monotremes, the Woodpeckers among Birds, and the Chamoeleon among Reptiles, the fact prob ably being that in countries where Termites and similar insects flourish various distinct forms of vertebrates have become modified in special relation to this abundance of nutritious food, which could only be made available by a peculiar structure of the alimentary organs. A close study of the more essential portions of the anatomy of these animals 2 leads to the belief that all the American Edentates at present known, however diversified in form and habits, belong to a common stock. The Bradypodidx, Megathe- riidx, and Myrmecophagidx are closely allied, the modifica tions seen in the existing families relating only to food and manner of life. The ancestral forms may have been omni vorous, and gradually separated into the purely vegetable and purely animal feeders ; from the former are developed the modern Sloths, from the latter the Anteaters. The Ar madillos (Dasypodidse) are another modification of the same type, retaining some generalized characters, as those of the alimentary organs, but in other respects, as their defen sive armature, remarkably specialized. The two Old- World families Manidse, and Orycteropodidos. are so essentially dis tinct, both from the American families and from each other, that it may even be considered doubtful whether they are derived from the same primary branch of mammals, or whether they may not be offsets of some other branch, the remaining members of which have been lost to knowledge. Family BRADYPODID.E. Externally clothed with long, coarse, crisp hair. Head short and rounded. External ears inconspicuous. Teeth in each jaw, snbcylindrical, of persistent growth, consisting of a central axis of vaso-dentine, with a thin investment of hard dentine, with a thick outer coating of cement ; without (as far as is yet known) any suc cession. Fore limbs greatly longer than the hind limbs. All the extremities terminating in narrow, curved feet ; the digits never FIG. 34. Skull of Two-toed Sloth (Cholcepus didactylus). From Proc. Zool. Soc., 1871, p. 432. exceeding three in number, encased for nearly their whole length in a common integument, and armed with long strong claws. Tail rudimentary. Stomach complex. No caecum. Placenta dccid- uate, dome-like, composed of an aggregation of numerous discoidal lobes. Strictly arboreal in habits, vegetable feeders, and limited geographically to the forest regions of South and Central America. Two genera, Bradypus and Cholcepus. See SLOTH. Family MEGATHEK.IID.E. The members of this family are all extinct. Their charac ters, so far as is known from the well-preserved remains of many species found abundantly in deposits of Pleistocene age in 2 See Proceedings of the Zoological Society of London, 1882, p. 358.