CHAPTER XVI

HISTORY OF THE EDENTATA

As here employed, excluding the so-called edentates of the Old World, the Edentata form a highly variegated, but natural, assemblage of related forms. The order is at present exclusively American and almost confined to the Neotropical region, an armadillo which extends into Texas being the sole exception. These animals are so peculiar and so isolated from other mammals, that it has been proposed to treat them as a separate subclass; and there is much to be said in favour of this procedure, though it would perhaps be premature, until more is learned concerning these most curious and exceptional animals. In the subjoined table only the more important and better known genera are included.

Series PILOSA. Hairy Edentates

Suborder TARDIGRADA. Tree-Sloths

I. BRADYPODIDÆ.

_Bradypus_, Three-toed Sloth, Rec. _Cholœpus_, Two-toed Sloth, Rec.

Suborder VERMILINGUA. Anteaters

II. MYRMECOPHAGIDÆ.

_Myrmecophaga_, Ant-Bear, Rec. _Tamandua_, Lesser Anteater, do. _Cyclopes_, Tree Anteater, do.

Suborder †GRAVIGRADA. †Ground-Sloths

III. †MEGATHERIIDÆ.

_†Megatherium_, Plio. and Pleist., S. A.; Pleist., N. A. _?†Prepotherium_, Santa Cruz. _?†Planops_, do.

IV. †MYLODONTIDÆ.

_†Mylodon_, Plio. and Pleist., S. A.; Pleist., N. A. _†Paramylodon_, Pleist., N. A. _†Grypotherium_, Pleist., S. A. _†Pseudolestodon_, Plio. and Pleist., S. A. _†Scelidotherium_, do. _†Nematherium_, Santa Cruz. _†Analcitherium_, do.

V. †MEGALONYCHIDÆ.

_†Megalonyx_, Pleist., N.A. _†Nothrotherium_, Pleist., S. A. _†Megalocnus_, Pleist., Cuba. _†Hapalops_, Santa Cruz. _†Schismotherium_, do. _†Pelecyodon_, do. _†Megalonychotherium_, do. _†Protobradys_, Casa Mayor.

Series LORICATA. Armoured Edentates

Suborder DASYPODA. Armadillos

VI. DASYPODIDÆ.

_Dasypus_, 6-, 7- and 8-Banded Armadillos, Pleist. and Rec., S. A. _Cabassous_, 11-Banded Armadillo, do. _Priodontes_, Giant Armadillo, do. _Tolypeutes_, Apar, Rec., S. A. _Zaëdyus_, Pygmy Armadillo, do. _Scleropleura_, do. _Chlamydophorus_, Pichiciago, do. _Tatu_, 9-Banded Armadillo, Pleist. and Rec., S. A.; Rec., Texas. _†Eutatus_, Plio. and Pleist., S. A. _†Chlamydotherium_, do. _†Proeutatus_, Santa Cruz. _†Prozaëdius_, Deseado and Santa Cruz, _†Prodasypus_, do. _†Stegotherium_, Santa Cruz. _†Meteutatus_, Deseado. _†Sadypus_, do. _†Amblytatus_, do. _†Prœuphractus_, do.

VII. PELTEPHILIDÆ.

_†Peltephilus_, Deseado and Santa Cruz.

VIII. INCERTÆ SEDIS.

_†Metacheiromys_, mid. Eoc., N. A.

Suborder †GLYPTODONTIA. †Glyptodonts

IX. †GLYPTODONTIDÆ.

_†Glyptodon_, Plio. and Pleist., N. and S. A. _†Dœdicurus_, Pleist., S. A. _†Panochthus_, do. _†Sclerocalyptus_, Plio. and Pleist., S. A. _†Glyptotherium_, mid. Plio., N. A. _†Propalæohoplophorus_, Deseado and Santa Cruz. _†Cochlops_, Santa Cruz. _†Eucinepeltus_, do. _†Asterostemma_, do.

In the section Pilosa, which includes the sloths (Tardigrada), anteaters (Vermilingua) and the extinct †ground-sloths (†Gravigrada), the skin is thickly clothed with long hair, and in the Loricata, armadillos and †glyptodonts, the head, body, tail and legs are more or less completely encased in an armour of bony scutes covered with plates of horn, but with some hairs also.

The name Edentata (toothless) is not very happily chosen, for only the anteaters are quite toothless. Almost all the genera have no teeth in the front of the mouth and the teeth are nearly always alike, so that the distinction of regions among them is entirely a matter of position in the jaws. In the tree-sloths and many †ground-sloths the foremost tooth in each jaw is a more or less enlarged, canine-like tusk. The teeth are always rootless, growing from permanent pulps, and are without enamel, made up of dentine, which is sometimes homogeneous and sometimes in layers of different hardness, and with a covering of cement, usually thin and film-like. The number of teeth varies from 4/4 to 10/10 or more, and their form usually approximates a simple cylinder, worn off flat at the end, though the ends may be bevelled or grooved, differences which are in no way due to pattern but simply to the mode of wear. In the †glyptodonts the teeth were divided by deep vertical grooves into two or three pillars, connected by narrow necks. In most of the edentates there is no change of teeth, the milk-dentition having been completely suppressed, but in the 9-Banded Armadillo (_Tatu_) each of the permanent teeth is preceded by a two-rooted milk-tooth, and some other armadillos have milk-teeth.

The skull varies much in form and proportions, according to the character of the food and method of feeding. The tree-sloths and †ground-sloths have short, rounded heads; in the †glyptodonts, the skull was short and remarkably deep vertically; while the armadillos have long, shallow heads, with tapering muzzle, the length and slenderness of which differ in the various genera. In the anteaters the skull is extraordinarily elongate and slender. The sagittal crest is seldom present at all and never prominent. The zygomatic arch may be complete or interrupted; in the tree-sloths, †ground-sloths, †glyptodonts and some extinct armadillos, there is a descending, plate-like process given off beneath the eye.

The backbone displays some of the most remarkable peculiarities of the order. The neck in the tree-sloths has eight or nine vertebræ, the only instances known among mammals in which the normal number of seven is departed from. In the armadillos and †glyptodonts several of the neck-vertebræ are coössified into a single piece, but the atlas is always free, so as to permit the movements of the head. In the posterior part of the dorsal and in the lumbar region the articulations between the successive vertebræ are by far the most complex and intricate known among mammals; in the tree-sloths these have degenerated, though still plainly indicated. In the †glyptodonts, which were covered with a huge, tortoise-like carapace, mobility of the backbone was needless, and so all of the dorsal vertebræ were united into one long piece and the lumbars were coössified with one another and with the sacrum. The sacrum consists, throughout the order, of a very large number of vertebræ and is attached to the hip-bones at two different points, instead of only one, as in other mammals. The tail varies much in length and thickness; in the tree-sloths it is extremely short and in the anteaters very long and bushy, prehensile in the arboreal members of the group; in the †ground-sloths, especially the gigantic forms, it was of immense thickness; while in most of the †glyptodonts a varying number of the terminal vertebræ were fused together. The sternal ribs are better developed than in any other mammals, and in the anteaters and †ground-sloths they articulate with the breast-bone by regular synovial joints, and each rib has head and tubercle like a vertebral rib.

In the limbs and feet there is great variety, according to the manner of their employment. The shoulder-blade has a very long acromion and very large coracoid, which long remains separate from the scapula; collar-bones are very generally present, though often in much reduced condition. The hip-bones have in the tree-sloths, †ground-sloths and †glyptodonts a much expanded anterior element, which in the other groups is narrow. The humerus usually has very prominent deltoid and supinator ridges and epicondylar foramen; the fore-arm bones are always separate, and there is generally much freedom of rotation of the manus. In the wrist there is no distinct central and usually there are the ordinary eight separate bones. The tibia and fibula are frequently coössified. The tree-sloths, which lead most strictly arboreal lives and are almost helpless on the ground, are unique among mammals in that the body is habitually _suspended_ from the limbs, not carried upon them; the feet are curved hooks, which fit over the tree-branches and support the weight without muscular exertion. The limb-bones are very long and slender, the claws long, curved and sharp, and the metapodials of each foot, two or three in number, are fused into a single mass. In the †ground-sloths there was much change in foot-structure during the course of their recorded development; they were usually five-toed and the feet were armed with one or more great claws; the later and larger representatives of the suborder walked upon the outer edge of the feet.

The armadillos, which are largely burrowers, have five-toed feet and long, heavy, pointed claws, but in some of them the pes has a varying number of flat, hoof-like nails. The immense †glyptodonts had very short, broad feet, shod with hoofs, which, in some of the genera, were longer and more claw-like in the manus.

The recorded history of the edentates was developed almost entirely in South America. In the Casa Mayor formation there were numerous armadillos, but as only scutes of the carapace have been found, little is known of them. The †ground-sloths (_†Protobradys_) have been reported, but from such imperfect material that the reference is uncertain. The first assuredly determinable members of this suborder were in the Astraponotus beds and, associated with them, the most ancient known †glyptodonts. In the Deseado stage were many armadillos, some of them extremely peculiar, several †glyptodonts and †ground-sloths, some species of the latter very large. Edentates were far more numerous and varied in the Santa Cruz than in any of the preceding stages. Tree-sloths and anteaters have both been reported, but the evidence is insufficient, though there can be little doubt that these suborders had begun their separate existence in some part of South America other than Patagonia. The three families of †ground-sloths were already distinguishable, though much less clearly separated than they afterwards became; none of them were large animals, smaller even than some of the Deseado species and veritable pygmies in comparison with the giants of the Pliocene and Pleistocene. The †glyptodonts were likewise far smaller than their Pliocene and Pleistocene successors and in several respects more primitive, approximating the armadillos more closely; nor was there any such variety of forms as in the later stages. The armadillos were extremely numerous and varied; they all belonged to extinct genera and most of them apparently have no descendants at the present day. The tropical forests of Brazil and the Guianas must then, as now, have swarmed with mammals which did not extend their range to Patagonia and of which we consequently have no record. No doubt, it was in these forests that the ancestors of most modern armadillos, as well as of the tree-sloths and anteaters, lived in Miocene times.

Pliocene edentates were of the same suborders as those of the Santa Cruz, but far larger in size. Most of them are known only from very incomplete specimens, but the Pleistocene has yielded an enormous mass of beautifully preserved material. Of the tree-sloths and anteaters, only questionable remains have been found. That these tropical and forest-loving animals should not have occurred in the open Pampas of Argentina is not surprising, but it is difficult to account for their absence from the extremely rich cave-faunas of Brazil. Nearly all the existing genera of armadillos have been obtained, and with these were associated several extinct genera, some of them (_†Chlamydotherium_, _†Eutatus_) relatively huge, as large as tapirs. There was a wonderful variety of †glyptodonts, most of them enormous creatures, of which no less than five genera have been collected in Argentina and Brazil, and the †ground-sloths were even more numerous and varied. Nine genera, with many species, of these great beasts, which ranged in size from an elephant to a tapir, are already known and no doubt the list is still incomplete. These †glyptodonts and †ground-sloths must have been among the most conspicuous elements of the Pleistocene fauna.

Aside from certain problematical Eocene forms, the first North American edentates, which were immigrants from the southern continent, appeared probably in the middle Miocene of Oregon in the form of †ground-sloths, but the specimen, as well as a similar one from the lower Pliocene of Nebraska, is not sufficiently complete for positive reference. In the middle Pliocene the †ground-sloths and †glyptodonts were unquestionably present, and in the Pleistocene these two suborders were numerously and conspicuously represented. Three or four genera of the huge, elephantine †ground-sloths coexisted in Pleistocene North America. _†Megalonyx_ was abundant in the forested regions east of the Mississippi, from Pennsylvania southward, and on the Pacific coast; _†Mylodon_ was transcontinental in distribution; while _†Megatherium_ was apparently confined to the southern states. While all three genera undoubtedly originated in South America, _†Megalonyx_ has not yet been found in that continent.

This genus was originally named by President Jefferson in 1805 from an ungual phalanx found in a cave in Virginia, and he imagined that it belonged to a colossal lion which must still be living in the mountains of western Virginia. This was deduced from the assumption that no species could become extinct, and the passage is of interest as showing the prevalent belief of the time, although Cuvier had already demonstrated that many species had actually been extinguished. The passage is as follows: “The movements of nature are in a never ending circle. The animal species which has once been put into a train of motion is still probably moving in that train. For, if one link in nature’s chain might be lost, another and another might be lost, till this whole system of things should evanish by piecemeal.”

The †glyptodonts were also southern in distribution, and only very imperfect remains of them have yet been recovered from the North American Pleistocene, quite sufficient, however, to make the identification certain.

There were several genera of rather small †ground-sloths in the Pleistocene of Cuba. The best known of these, _†Megalocnus_, had several peculiarities of structure, but was plainly a member of the †Megalonychidæ. The ancestors of this genus probably invaded Cuba in the Pliocene, when the island was joined to Central America.

SUBORDER †GRAVIGRADA. †GROUND-SLOTHS

As the †ground-sloths would appear to have had a more central position within the order than any of the other groups, our study of development may well begin with them. In the Pleistocene there were three families of these gigantic brutes, which ranged through the western hemisphere from Pennsylvania and California to Patagonia. Unfortunately our knowledge of the developmental stages within the different families is very unequal, and it is therefore impracticable to do more than sketch the changes of the suborder as a whole and in a general way. In the successive geological stages the proportionate representation of the different phyla varied greatly; in the South American Pliocene and Pleistocene the †Mylodontidæ and †Megatheriidæ were the abundant forms, while the †Megalonychidæ were but scantily represented. In the Santa Cruz Miocene, on the other hand, the overwhelming preponderance was with the †Megalonychidæ, the other two families being comparatively rare and incompletely known. From the still more ancient formations, the material so far collected is so fragmentary that family distinctions have little meaning. After all, there was no very wide range of variation among the contemporary members of the three families, and the differences were principally in size, in the form and number of the teeth, the shape of the skull and the number of digits; in essentials they were all much alike.

The genus _†Megatherium_ (Fig. 122, p. 220) included the largest and most massive members of the suborder, _†M. americanum_ being as large as an elephant, but very differently proportioned, as it was much longer and lower in stature, owing to the shortness of the extraordinarily heavy limbs; some of the skeletons measure 20 feet or more in length. The teeth, which were 5/4 in number, formed an uninterrupted series on each side; all had the same quadrate form and by abrasion were worn into two transverse ridges, formed by the meeting of the harder dentine with the thick coating of cement. The result was a form of tooth which much resembled the lower molars of a tapir, but it was not a tooth-pattern in any proper sense of the word, being due entirely to the mode of wear.

The skull was very small in proportion to the huge body and was low and narrow in shape; the cranium had a broad, flat roof, without sagittal crest; the orbit was completely encircled in bone, and the descending process of the zygomatic arch beneath the eye was very long and conspicuous. The nasals were short, and the slender, toothless premaxillaries projected far in front of them, which makes the presence of some sort of a proboscis likely. The lower jaw had a long, narrow, spout-like symphysis, which was abruptly rounded at the free end, not pointed; below the teeth, the lower margin of the jaw was very strongly convex, descending in a great flange. The neck was short, the body very long and enormously heavy, as was also the tail. The immense shoulder-blade had a very long acromion, which curved forward and inward, fusing with the coracoid and forming a bony loop or bridge. The hip-bones had the anterior element (ilium) enormously expanded transversely, so as to support the huge mass of viscera in the semi-erect position which the animal, it is believed, frequently assumed in feeding. Collar-bones were present.

The fore limb was very much more slender than the hind, but of nearly the same length. The humerus had a comparatively slender upper portion and extremely broad lower end, due to the great development of the internal epicondyle and supinator ridge; there was no epicondylar foramen. The radius evidently had the power of very free rotation upon the humerus. The femur was short, flattened antero-posteriorly, but excessively broad and heavy, and had no third trochanter. The tibia and fibula were likewise short and very massive and were extensively coössified at each end, leaving but a short interspace open between the bones. The very peculiar feet were so connected with the limb-bones, that the animal must have walked upon the outer edge of the foot, somewhat as the existing Ant-Bear (_Myrmecophaga jubata_) uses the fore foot. The manus had four functional digits, the first being a mere vestige; the fifth, upon which the weight rested in walking, had two very small phalanges and no claw, while the second, third and fourth had long, sharp claws. The pes had but three functional digits, for the first and second were reduced to rudiments; digit III had an enormous claw and of this digit the metatarsal was short and very heavy and the first two phalanges were fused together; the two external digits, Nos. IV and V, had no claws. The astragalus had a very peculiar shape, made necessary by the application of the external border of the foot to the ground and thus in both fore and hind feet the great claws were turned inward and, in the case of the pes, it must have been impossible to rest the sole upon the ground. The heel-bone was enormous and club-shaped and formed the hinder portion of the weight-carrying outer edge of the foot.

Almost all who have studied the structure of this extraordinary beast are agreed as to its habits. That it fed principally, if not exclusively, upon leaves, is indicated by the teeth. The general opinion as to its manner of life is well summed up by von Zittel: “The hip-bones, hind legs and tail are characterized by enormous strength. The entire structure of the extremities proves that the gigantic sloth could move over the ground but slowly and clumsily; on the other hand, the fore limbs served as grasping organs and were presumably employed to bend down and break off twigs and branches and even to uproot whole trees, while the weight of the body was supported upon the hind legs and tail.”[19] It would be quite absurd to suppose that such ponderous animals could have been climbers or burrowers, hence the function of the enormous claws, especially the single one of the pes, is not obvious, though they may have been merely the weapons of the otherwise defenceless monsters. The great claw in the fore foot of the Ant-Bear is a terrible weapon, with which the creature vigorously and successfully defends itself against dogs, and it may even be dangerous to men, if incautiously molested.

_†Megatherium_ had no bony scutes, or other ossifications in the skin, so far as is known, and was probably covered with long and coarse hair, as is known to have been the case in another †ground-sloth.

Less specialized in many respects than the †megatheres was _†Mylodon_, type of a family which was numerously and variously represented in the Pleistocene of South America, much less so in that of North America. _†Mylodon_ was smaller and lighter, being from ⅓ to ¼ smaller in linear dimensions than _†Megatherium_, and the contemporary _†Scelidotherium_ was no bigger than a tapir. The teeth numbered 5/4 and the anterior one above and below had a somewhat tusk-like form; the others were worn off evenly, with nearly horizontal grinding surface, but a vertical groove on the inner side gave them a subtriangular, lobate form. The skull was short and broad, with flat top, and orbit only partially enclosed behind; the premaxillaries were very short and the muzzle very broad and abruptly truncated, the nasal opening very large. The lower jaw had a straight inferior border, a short, very wide and shovel-shaped symphysis and square chin. Nothing indicates a proboscis, and the head must have been very different from that of _†Megatherium_.

Within the family of the †mylodonts there was some variety in the dentition and more in the shape of the skull. In _†Lestodon_, for example, the first tooth in each jaw was a large, sharp-pointed tusk, the muzzle was greatly broadened, and the whole animal was larger. _†Scelidotherium_, the smallest Pleistocene member of the family, had a much narrower and more elongate skull than the others. In _†Glossotherium_, which also had an elongate skull, there was an arched bony bridge connecting the anterior end of the nasal bones with the premaxillaries and dividing the nasal opening into two parts.

The neck, body and tail of _†Mylodon_ did not differ materially from those of _†Megatherium_, except in being smaller and less massive. The fore limb was relatively somewhat shorter and much stouter, but otherwise similar; the humerus had no epicondylar foramen and the femur no third trochanter; the tibia and fibula were separate. The manus had five digits, Nos. I, II and III carrying claws, that of III being especially large; IV and V had no claws and the outer edge of the manus rested on the ground in walking, the sole turned inward. The pes had lost the first digit, the second and third had claws, but not the fourth and fifth; the weight rested on the outer edge.

The skin is definitely known from large pieces belonging to the allied genus _†Grypotherium_, found in a cavern near Last Hope Inlet, Patagonia, where it had been preserved by burial in dry dust. Externally, the skin was thickly covered with coarse hair and in the deeper layers was a continuous armour of small ossicles, which were close set and in the Last Hope specimens show like a cobble-stone pavement on the inner side of the skin, the innermost layers of which have been destroyed; in life, these small bones were not visible. Similar ossicles have been found in association with several skeletons of _†Mylodon_. The habits, diet and mode of feeding of the latter were no doubt essentially similar to those of _†Megatherium_, but _†Scelidotherium_, which had a much shorter and lighter tail, was probably more quadrupedal and browsed upon low shrubbery.

[Illustration: FIG. 288.—Gigantic †ground-sloth (_†Mylodon robustus_), Pampean. Restored from Owen’s figure of the skeleton.]

The third family, the †Megalonychidæ, was scantily represented in the Pleistocene of South America, but relatively common in North America. _†Megalonyx_ was, on the whole, less specialized than _†Mylodon_ or _†Megatherium_, but had a strong resemblance to both of them. The teeth, 5/4 in number, had the foremost one in each jaw separated by a considerable space from the others and more or less tusk-like in form; the grinding teeth were worn smooth, without ridges, and of somewhat trihedral shape. The skull was short, broad and deep, resembling in shape that of the tree-sloths; there was a long, but feebly developed sagittal crest, and the orbits were widely open behind, with hardly a trace of any posterior boundary. The muzzle was very short and broad and abruptly truncated and the premaxillary bones were extremely small. The lower jaw was short, thick and massive, with very broad symphysis and almost vertical chin. Neck, body, tail, shoulder and hip-bones did not differ sufficiently from those of _†Megatherium_ to require particular notice.

The fore limb was shorter and more slender than the hind; the humerus had the epicondylar foramen and the very massive femur retained the third trochanter; the tibia and fibula were separate. The feet had five digits, three of which carried claws; the calcaneum was very peculiar, not at all like the massive, club-shaped bone of _†Megatherium_ and _†Mylodon_, but long, comparatively thin and sickle-shaped. Nothing in the skeleton suggests that the creature’s habits differed in any important way from those of the genera last named.

_†Megalocnus_, of the Cuban Pleistocene, a member of this family, was apparently peculiar to the island and was probably derived from ancestors which in the Pliocene migrated from Central America. Aside from certain remarkable peculiarities of the teeth, this animal was more primitive, as well as smaller, than any other of the Pleistocene genera.

Although remains of †Gravigrada are comparatively common in all of the fossiliferous formations between the Pampean and the Santa Cruz, the material is too imperfect to throw any useful light upon the development of the various families. From the Santa Cruz beds, on the other hand, a great wealth of specimens has been obtained, and it is possible to give some fairly adequate account of the †ground-sloths of that time. These animals were then extremely abundant individually and of extraordinary variety; evidently, they were in a state of rapid expansion and divergent evolution along many lines, for hardly any two specimens are alike and therefore the satisfactory discrimination of species is well-nigh impossible. Yet, with all this remarkable variability, the range of structural differences was not great; the group was a very homogeneous and natural one, and separation into families was not obvious. Two of the three families were, however, unequivocally present in this fauna and the third somewhat doubtfully so. The †Megalonychidæ, which in the South American Pleistocene had dwindled to such insignificant proportions, formed the overwhelming majority of the Santa Cruz †Gravigrada; the †Mylodontidæ were quite rare in comparison and are still very incompletely known; while the †Megatheriidæ, though probably present, have not been identified beyond all doubt.

All of the Santa Cruz †ground-sloths were small animals, the largest not approximating the smallest Pleistocene species, those of Cuba excepted, and many of them were no larger than the modern tree-sloths. This was a wonderful difference between the Santa Cruz and the Pampean, but a difference which involved nearly all other groups of mammals. So far as the skeleton is concerned, this is known with completeness only for the †Megalonychidæ, especially the genus _†Hapalops_; but enough has been learned of the others to show that there was far less difference between the families than had arisen in the later epochs. This backward convergence of the three groups towards a common term plainly indicates their common origin, being exactly what might have been predicted in advance of experience.

[Illustration: FIG. 289.—Santa Cruz †ground-sloth (_†Hapalops longiceps_) and †glyptodont (_†Propalæohoplophorus australis_). Restored by Knight from skeletons in Princeton University and the museum of La Plata.]

In all the genera the teeth number 5/4; the teeth on each side were sometimes in continuous series, sometimes the first one was isolated and almost always more or less tusk-like, most so in _†Eucholœops_. The other teeth were usually of transversely elliptical shape and worn into two ridges, with a hollow between; the †mylodonts (_†Nematherium_, etc.) already had the triangular, or lozenge-shaped, lobate form of teeth, characteristic of the family.

The skull varied considerably in its proportions; generally, it was long and narrow, with shortened face and elongate cranium; the sagittal crest was seldom present, never prominent, and the orbit was always widely open behind, without postorbital processes. The premaxillaries were always short and toothless and in most of the genera they were slender rods, in others (_e.g._ _†Hyperleptus_) broad and plate-like. The lower jaw had an elongate spout-like symphysis, in which the two halves were coössified, tapering forward to a blunt point and, though the length of this spout differed greatly in the various genera, in none was there a broad, abrupt chin such as _†Mylodon_ and _†Megalonyx_ had. In _†Prepotherium_, which is believed to be referable to the †Megatheriidæ, the lower jaw had the extremely convex inferior border, in less exaggerated degree, of its huge Pampean successor; it would be premature to say descendant.

While the long, slender skull was the prevailing type among the Santa Cruz †Gravigrada, there was a group of small animals in which the skull was shorter and more rounded and had a very suggestive likeness to that of the modern tree-sloths, as was likewise true of the teeth.

Despite innumerable variations of detail, the skeleton of the Santa Cruz †ground-sloths may be described without distinction of genera, though it should be added that the skeleton is but partially known in many of the genera, and fuller knowledge might require modification of some of the statements. The neck was of moderate length, the body long, the tail long and heavy and, in some instances, very massive. The sternal ribs were completely ossified and already had the same elaborate mode of articulation with the breast-bone as in the great Pampean forms, and the vertebræ the same intricate connections. The shoulder-blade also had the same characteristics as in the latter, but the hip-bones had but a moderate transverse expansion, having no huge mass of viscera to support.

The limbs were stout and short, fore and hind legs of nearly equal length; the humerus had the epicondylar foramen and the broad, flattened femur retained the third trochanter. The radius had a discoidal upper end, which rotated freely upon the humerus; the tibia and fibula were always separate. The feet were five-toed, all the digits complete and functional and all provided with claws; there was no coössification between the phalanges. The astragalus was little different from the normal form, but in some genera (_e.g._ _†Prepotherium_) the highly peculiar form of this bone characteristic of _†Mylodon_ and _†Megatherium_ was distantly foreshadowed. The gait must have been simply plantigrade, though some of the forms had probably begun to throw the weight upon the outer edge of the foot.

[Illustration: FIG. 290.—Left pes of _†Mylodon_, Pampean (after Owen). _Cal._, calcaneum. _As._, astragalus. _N._, navicular. _Cn. 2_, _Cn. 3_, middle and external cuneiforms. _Cb._, cuboid.]

No dermal armour has yet been found in association with any of the genera, and, so far as the predominant †Megalonychidæ are concerned, of which so many skeletons have been collected, this negative evidence must be allowed great weight. But the material of the other two families is so rare and incomplete, that the failure to find dermal ossicles is of no value in determining the question; probably, the †mylodonts possessed them.

These small Santa Cruz †ground-sloths were not so clumsy and slow-moving as their gigantic successors of the Pampean, and must have been inoffensive plant-eaters, some of them perhaps more or less arboreal in habits, but they could defend themselves with their long, sharp claws.

[Illustration: FIG. 291.—Left pes of _†Hapalops_, Santa Cruz. Princeton University Museum. Letters as in Fig. 290 and scale of reduction the same.]

It would require far too much space and lead us into a labyrinth of anatomical technicalities to point out all the many resemblances to other edentate suborders which are to be noted in the skeleton of the Santa Cruz †Gravigrada, which thus justified their position as the most nearly central group of the entire order. Not only was the skeleton of these Miocene †ground-sloths very much less specialized than in their Pleistocene successors, but they were much closer to the anteaters than were the latter. Aside from the skull, all parts of the skeleton displayed this resemblance in so marked a manner that the common derivation of the two suborders seems hardly open to question. Different as was the skull in the two groups, the differences were not such as to preclude the origin of both from the same type. Even more closely connected were the †ground-sloths and the tree-sloths; the resemblance was most clear in the teeth and skull, but there were also many points of likeness throughout the skeleton. In the tree-sloths the entire bony structure has been profoundly modified in adaptation to their altogether exceptional mode of life, in hanging _suspended_ from the branches of trees; but, despite this modification, there are so many notable resemblances between the Santa Cruz †Gravigrada and the existing Tardigrada as irresistibly to suggest their community of origin, and thus the former served to connect the anteaters, on the one hand, with the tree-sloths, on the other. This must not be construed as meaning that the Miocene †ground-sloths were the ancestors of the other suborders, which were probably already in existence as distinct groups, but merely that all three suborders had a common origin, from which the Santa Cruz †Gravigrada had departed less than have the sloths and anteaters.

There is evidence that at least two of the †ground-sloth families, the †Megalonychidæ and the †Mylodontidæ, were distinguishable in the Deseado stage, but materials are still lacking to give us any real knowledge of the suborder in that or the more ancient stages.

SECTION LORICATA. ARMOURED EDENTATES

SUBORDER DASYPODA. ARMADILLOS

Armadillos are still an important and characteristic element of the Neotropical fauna, ranging from Texas to Patagonia and showing a considerable variety of structure and appearance. Existing species are all of small or moderate size, and the one which is by far the largest (_Priodontes gigas_) may somewhat exceed three feet in length, exclusive of the tail, and the smallest (_Chlamydophorus truncatus_) is hardly more than five inches long. In most armadillos the hair is greatly diminished in quantity and the animal is sheathed in a conspicuous armour of bony scutes, covered with horny plates. There is a head-shield which covers the top of the skull, and the tail is enclosed in a sheath; the back and sides are protected by the great carapace and the limbs by irregular scutes and scales, leaving only the under side of the body and the inside of the legs uncovered. In most existing genera, the carapace is in three parts, an anterior and posterior buckler, in which the plates are immovably fixed together by their edges, and between a varying number of transverse, overlapping bands, from 3 to 13, which permit sufficient flexibility of the body. The tail-sheath is made up of a series of rings. One genus (_Tolypeutes_) has the power of rolling itself into a ball, the head-shield exactly closing the anterior notch of the carapace and the tail-sheath filling the posterior notch. The animal is thus perfectly protected against attack and does not seek refuge by digging, as other armadillos do and with astonishing rapidity. In the little Pichiciago (_Chlamydophorus_) the dermal ossifications are very thin and the carapace is composed of twenty transverse bands of horny plates, without bucklers; the rump is covered with a broad and heavy shield of bone, overlaid with thin plates of horn, which is attached to the hip-bones and notched below for the short tail. In certain rare and little known genera there is a greater development of hair; in one (_Praopus_) the whole carapace is covered with a dense coat of hair, and in another (_Scleropleura_) the middle of the back has only a hairy skin and the carapace is restricted to the sides.

The teeth vary in number and size in the different genera; in some (_e.g. Dasypus_) there is one upper incisor on each side; the teeth are all simple and of nearly cylindrical form. The skull is low and flattened, with long tapering snout and orbits widely open behind; the zygomatic arches are uninterrupted. Most of the vertebræ of the neck are fused into a single piece; in the lumbar and posterior dorsal regions there are not only the usual highly intricate articulations between the vertebræ, but also high processes on each side for the support of the carapace. The fully ossified sternal ribs have movable joints with the breast-bone, but not the double articulations found in the anteaters and †ground-sloths. The shoulder-blade has a very long acromion, which does not form a bony loop with the coracoid, and the clavicles are complete. The anterior element (ilium) of the hip-bone is narrow, very different from the broad plate of the †Gravigrada. The humerus has prominent deltoid and supinator ridges and an epicondylar foramen, and the femur has the third trochanter. Though the fore-arm bones are separate, the radius has no freedom of rotation; tibia and fibula are coössified at both ends.

In the hind foot there is no great variety of character; it is five-toed and usually has claws, but may have broad, flat nails (_e.g. Priodontes_), but the manus, which is a burrowing organ, displays different degrees of specialization, which is carried farthest in the Giant Armadillo (_Priodontes_). _Tatu_ has the fore foot of quite different type. The armadillos feed chiefly upon insects and worms, but they are omnivorous and eat roots and carrion and sometimes even capture and devour small rodents and lizards.

As in the case of the †ground-sloths, the fossil armadillos so far available are insufficient for tracing the history of the various phyla, or for doing more than making a very brief sketch of the development of the suborder as a whole. Nearly all of the modern genera have been found in the Pleistocene together with several that are extinct, some of the latter of very large size. One of these, _†Eutatus_, had a carapace without bucklers and made up of 33 movable, transverse bands. Another, _†Chlamydotherium_, as large as a rhinoceros and the largest known armadillo, had anterior and posterior bucklers, with several movable bands between; it was especially characterized by the teeth, which were divided by a vertical groove into pillars or lobes, thus approximating the teeth of the †glyptodonts. The genus went far back into the Pliocene, and the more ancient species were successively smaller.

Though remains of armadillos abound in the formations between the Pampean and the Santa Cruz, they are for the most part so fragmentary as to be of no service in deciphering the history of the group. In the Santa Cruz beds also they are very abundant and varied, and several of the genera are very completely known. As a whole, this assemblage of armadillos was very different from that of the Pleistocene, and only a few direct ancestors of the latter have been found in the Miocene of Patagonia; no doubt, like the ancestral tree-sloths and anteaters, they were then living in the warmer regions of the north. Most of the Santa Cruz armadillos belonged to aberrant types, of which no descendants have survived; but, nevertheless, they throw welcome light upon the developmental stages of the suborder.

[Illustration: FIG. 292.—Skull of _†Peltephilus_, Santa Cruz. Ameghino collection.]

These armadillos had the complete armour of head-shield, carapace and tail-sheath, but the carapace had no anterior buckler in any of the Santa Cruz genera, and in some there was no posterior buckler, the carapace consisting entirely of transverse, movable bands, as in the Pleistocene _†Eutatus_. In one especially peculiar genus, _†Peltephilus_, the head-shield was remarkable; it was made up of large, polygonal plates, the two anterior pairs of which were elevated into high, sharp points, which must have supported horns, that were quite large in proportion to the size of the animal. A 4-horned armadillo, like a tiny rhinoceros in armour, must have been a sufficiently bizarre object.

As a rule, the teeth of the Santa Cruz armadillos were of the same simple, cylindrical form as in the modern genera and arranged in the same way, but there were some exceptions. In the horned _†Peltephilus_, the teeth of each jaw were so inserted as to form a continuous series around the sides and front of the mouth; and, at first sight, it would seem that this genus differed from all other known edentates in having a full set of incisors, but actually it had but one on each side above and below, as has the modern _Dasypus_, with the difference that, in the latter, the incisors of the opposite sides are widely separated and in _†Peltephilus_ were brought close together. The anterior upper teeth were long and sharp and passed outside of the lower ones, when the jaws were closed, and all the teeth had an external layer of hard and shining dentine, which had almost the appearance of enamel. Another variant in dentition was _†Proeutatus_, which was the largest of Santa Cruz armadillos and larger than any existing forms except _Priodontes_ and _Cabassous_. It had teeth like those of the huge Pliocene and Pleistocene _†Chlamydotherium_, of which it was a probable ancestor; the five posterior ones in each jaw were of trihedral shape, and the two kinds of dentine, of which they were composed, were so arranged as to form a rough grinding surface. Probably this animal subsisted largely upon vegetable food; at all events, the food was of such a nature as to keep the teeth worn down more than in any of the associated genera. A fourth type of dentition was displayed by _†Stegotherium_ (Fig. 243, p. 480); the teeth were so few and small that they can have had no functional value and were merely minute points almost level with the gums. In all probability, _†Stegotherium_ was more exclusively insectivorous than the other genera.

Among the Santa Cruz armadillos may be distinguished four well-marked types of skull. (1) That which agrees closely with the modern form, especially as exemplified by the genus _Dasypus_. (2) _†Proeutatus_ had a higher and less flattened cranium and a very long, cylindrical muzzle. (3) In the horned _†Peltephilus_ the face was very short and broad, and the lower jaw was horseshoe-shaped, the two halves coössified at the symphysis, which is not true of any other armadillo. (4) Quite the opposite extreme was displayed by _†Stegotherium_, in which the face was drawn out into a very long, slender and tapering muzzle; the lower jaw was extremely weak and thin, the posterior, ascending portion low and ill-defined, the condyle and coronoid process much reduced. No other known armadillo has such fragile jaws, and there was a distinct likeness in the skull to that of the Ant-Bear.

[Illustration: FIG. 293.—Skull of _†Proeutatus_, Santa Cruz. Princeton University Museum.]

[Illustration: FIG. 294.—Skull of _†Stegotherium_, Santa Cruz. Princeton University Museum.]

Aside from carapace and skull, the skeleton of the Santa Cruz armadillos was surprisingly modern. The vertebræ of the neck were coössified, those of the lumbar and posterior dorsal regions had the extremely complex articulations and the high processes for the support of the carapace, just as in the Recent genera. The limb-bones did not differ in any significant way from those of the latter, and the feet closely resembled those of the modern _Dasypus_; none of the genera displayed the specialization of the manus seen in _Cabassous_, _Priodontes_ or _Tolypeutes_. Whether these specializations have all been acquired since Santa Cruz times, or whether they had already appeared in some other region of the continent, is a question that remains to be determined.

Little can yet be done in the way of tracing the history of the armadillos through the stages preceding the Santa Cruz times, because of the fragmentary character of the material. The suborder was abundantly represented in the Deseado stage, in which some of the Santa Cruz genera existed. Even in the most ancient of the Patagonian Tertiary formations are found scutes of the carapace essentially like those of the modern armadillos. The group is thus of very high antiquity, older than any other of the suborders is known to be.

In addition to the typical armadillos of South America, there were, in other continents, certain more or less doubtful forms, concerning which a word should be said. In the Bridger Eocene of North America was a genus (_†Metacheiromys_) of armadillo-like animals, the true relationships of which are far from clear. The teeth were mostly lost, leaving but one on each side of each jaw, and this _was covered with enamel_, which is not true of any unquestioned edentate. However, this is not an insuperable objection to the inclusion of these animals in the edentates, for there can be no doubt that these were derived from ancestors with enamel-covered teeth. Even in modern armadillos the enamel-organ is formed in the embryo, though it does not perform its functions. The skull of _†Metacheiromys_ had something of the armadillo-shape, but was not especially characteristic. The vertebræ of the neck were all separate, and those of the dorsal and lumbar regions did not have the complex articulations common to all known edentates, fossil and Recent; the sacrum had on each side but one point of contact with the hip-bones, and the sternal ribs were not ossified. The shoulder-blade, hip-bones and humerus were all armadillo-like. The plantigrade feet were five-toed and the metapodials were very edentate in form. No indication of bony armour has been found. While these curious animals may very possibly have been referable to the Edentata and, at all events, had several features suggestive of relationship to that order, it can hardly be maintained that they were unequivocal members of it. In the Oligocene of France have been obtained some very fragmentary fossils which were classified and described as armadillos, but their character is quite problematical. It is thus possible, though far from certain, that in the early Tertiary, armadillo-like edentates were spread all over the northern hemisphere.

SUBORDER †GLYPTODONTIA. †GLYPTODONTS

In the Pliocene and Pleistocene these huge armoured creatures ranged from the southern United States to Patagonia. That they were nearly related to the armadillos is clear, but they were so greatly modified and specialized as to demand recognition as a distinct suborder.

Aside from their enormous size, the most striking feature of the †Glyptodontia is the extraordinary development of their defensive armour, which was far more complete and massive than in the armadillos. The top of the head was protected by a thick head-shield, or _casque_, composed of several coössified plates; the body and much of the limbs were enclosed in the immense carapace of elongate-oval, domed shape, which covered the neck and trunk and on the sides almost reached to the ground. This tortoise-like carapace was composed of very thick, polygonal plates of bone (no doubt covered externally with horny plates) immovably fixed together by their rough edges, and ornamented with an elaborate pattern of sculpture, which varied according to the genus. With one or two exceptions, the plates of the carapace were not arranged in transverse rows, but formed a mosaic without discernible banding. In the exceptions noted, the sides of the carapace were made up of bands, and near the margins were two or three overlapping transverse bands which permitted a minimal degree of flexibility. The tail-sheath was remarkable and differed much in appearance and make-up in the various genera. In _†Glyptodon_ the tail was comparatively short and the tail-sheath was made up of a series of overlapping rings, each ring consisting of two rows of plates; those of the second row were ornamented, on the top and sides of the tail, with very prominent, conical projections, capped, in the living animal, with still longer and sharper spines of horn, so that the tail must have bristled with spikes. A more usual type of tail-sheath was exemplified by _†Sclerocalyptus_, in which there were several overlapping rings at the root of the tail, but for much the greater part of its length the plates of the sheath were fused together into a long, transversely oval tube, tapering very gently to the free end, where it was bluntly rounded. A modification of this type was the very long tail-sheath of _†Panochthus_, in which there were seven overlapping rings at the root, followed by a long, massive tube, the sides of which were set with three or more large and heavy, horn-like spines. In _†Dœdicurus_ was reached the maximum specialization of this type; the very long tube had its free end greatly expanded and thickened into a huge, club-shaped mass, on the top and sides of which were fixed long and sharp horns.

The teeth, which in all the known genera numbered 8/8, were all very much alike; each was divided by two broad and deep vertical grooves on each side into three pillars, connected by narrow necks. Harder dentine in the centre and on the periphery of the tooth, with a softer intermediate layer, kept the grinding surface rough through differential wear. Teeth of this character are indicative of a vegetable diet and these great creatures were, no doubt, as harmless and inoffensive as possible.

[Illustration: FIG. 295.—Pampean †glyptodonts, _†Dœdicurus clavicaudatus_ and _†Glyptodon clavipes_. Restored from skeletons in the museums of La Plata and Buenos Aires.]

The skull was remarkably short, broad and high, the facial region being especially abbreviated; the cranium, though forming the greater part of the skull, was yet small in comparison with the size of the animal; it had a distinct, though not prominent, sagittal crest. The occipital surface was inclined forward and had a very elevated position, the condyles being near the top of the head and raised very far above the level of the teeth. The orbits were relatively small, more or less completely encircled with bone and as near to the top of the head as they could be brought; this was to make room for the extremely high teeth, which required a great depth of jaw; the elevation of the whole cranium left unlimited space for the jaws beneath it. The zygomatic arches were strong and curved out widely from the sides of the skull; beneath each eye was given off a very long descending process which projected downward, outside of the lower jaw. In most of the species the upper profile of the skull was nearly straight, but in _†Panochthus_ it descended very steeply from the forehead to the nose. The forehead was dome-like and the nasals extremely short. Sinuses were extensively developed, especially in the frontals, and in _†Sclerocalyptus_ the bones around the nostrils were grotesquely inflated. The two halves of the lower jaw were fused together, and the symphysis was prolonged into a short, wide spout, which projected considerably in advance of the upper jaw, showing that the soft parts of the muzzle must have had a corresponding extension. The horizontal portion of the lower jaw, carrying the teeth, was short and very deep; the posterior, ascending portion had a forward inclination and was very high.

The skeleton of the Pleistocene †glyptodonts was unique among mammals, though evidently a modification of the armadillo type. The extreme modification was conditioned by the enormous weight of the carapace, which the skeleton had to support. The neck was very short, made up of short vertebræ, which were extensively coössified; the atlas was always free, but the axis was fused with a varying number of the succeeding vertebræ; usually, the axis and the third to the sixth formed one mass, while the seventh was fused with the dorsals. The joint between the sixth and seventh vertebræ was such as to permit at least a partial downward bending of the head beneath the carapace, closing its anterior opening with the head-shield. The seventh neck vertebra and all the dorsals, except the last one, were coössified into a heavy curved rod, the “dorsal tube”; the conjoined neural arches formed a tunnel for the spinal cord and the spines made a continuous ridge. As the hind legs were very much longer than the fore, the back was strongly arched upward from the neck to the hips. The last dorsal, the lumbars and the sacrum were all fused together to form the “lumbo-sacral tube,” of which the coössified neural spines made a very prominent ridge, the principal support of the carapace in the median line; the anterior half of the trunk skeleton, comprising the short, deep thorax, was free from the carapace, which in that region must have rested upon the muscles of the back and shoulders. The number of neck and trunk vertebræ combined varied in the different genera from 26 to 28, but fusion had reduced the number of separate parts to 4, or at most 5. Such greatly diminished flexibility of the back was rather an advantage. The tail differed much in length in the various genera, but was always massive; the anterior vertebræ, usually 7 in number, were free, the others were fused into a heavy, tapering rod; but for nearly its whole length the processes of the vertebræ were very prominent, each vertebra touching the tail-sheath at five points and thus giving it very effective support. In _†Glyptodon_ the tail-vertebræ were all free.

In most of the genera the scapula was very broad and had the very long acromion common to all the edentates; there were no clavicles. The hip-bones were very peculiar; the anterior element (ilium) stood almost vertically, at right angles to the backbone, and formed a broad plate, facing forward, the top of which was roughened and thickened to support the carapace. The posterior element (ischium) was also much expanded, but faced outward, and its hinder end, curved upward and thickened, was another point of strong support for the carapace. The two elements together formed an inverted arch, the crown of which rested on the head of the femur.

Though less massive than those of the hind leg, the bones of the fore limb were yet very heavy. The humerus was short and had reduced deltoid and supinator ridges and no epicondylar foramen; the short fore-arm bones were separate and heavy, the ulna especially so. The femur was much the longest of the limb-bones and was extremely strong, especially in its great breadth, the antero-posterior flattening, common to nearly all very heavy mammals, being well marked. A very unusual feature was the position of the third trochanter near the lower end of the shaft. The tibia and fibula were much shorter than the femur, extremely heavy and coössified at both ends. The very short and broad feet retained five digits; in the manus the claws were sometimes comparatively long and sharp, sometimes blunt and hoof-like; those of the hind foot were always broad hoofs.

Among all the many strange and grotesque mammals which the study of fossils has brought to light, none can have been more remarkable than the Pleistocene †glyptodonts; slow-moving hillocks they must have seemed, the larger species 12 to 14 feet long and 5 feet or more in height. Those that had claws on the fore feet probably used them to dig for roots and tubers, but all were plant-feeders. When attacked by the †sabre-tooth tigers (_†Smilodon_) or the great bears (_†Arctotherium_) they needed only to squat down, bringing the edges of the carapace to the ground, and draw in the head, to be perfectly protected, while a sweep of the spiny or club-like and horned tail would have been fatal to anything in its path.

As in the case of so many other groups, little has yet been learned regarding the history of the †glyptodonts during the interval between the later Pliocene and the Santa Cruz; the intermediate formations have yielded many †glyptodonts, but not in such preservation as to be of any service in this connection. We find, as might be expected, many and very great differences between the Pampean and the Santa Cruz representatives of the suborder, the latter being in all respects less modified and less widely removed from the armadillos.

(1) The most obvious and striking distinction was in size, the Santa Cruz forms being all small and some of them very small.

(2) In all cases the carapace was made up of transverse bands, which permitted a slight degree of flexibility, and near the anterior end, at the margins of the shell, were two or three overlapping bands. The plates were thin and were but rarely coössified; the ornamentation was made by shallow grooves.

(3) The tail-sheath, which was of very uniform character, consisted of two quite distinct portions; the anterior region consisted of 5 or more freely movable, overlapping rings, each of two rows of plates, and in the posterior region the rings were closely fitted together, less distinctly marked and not movable. This posterior portion was sometimes thick and ended abruptly, sometimes slender and tapering and in one genus (_†Asterostemma_) it was very armadillo-like. In none of the genera were there any spines or horns, nor were the separate plates ever fused together to form a tube.

(4) There was considerable variety in the head-shield, which was usually made up of many separate plates, but in one genus (_†Eucinepeltus_) they were coössified into a single heavy casque.

(5) The teeth had a less extreme height and the four anterior ones of each jaw were much simpler than in the Pampean forms. An interesting survival was the retention of two minute incisors in each premaxillary bone, in one genus (_†Propalæohoplophorus_), but these were of no functional value and were early lost.

(6) The skull was much longer, narrower and lower and had a relatively longer facial portion; the occiput was higher and more erect, and the condyles had no such elevation above the level of the teeth; the orbit was widely open behind and the descending process given off from the zygomatic arch beneath the eye had no such exaggerated length; the bones were not conspicuously inflated by sinuses. The lower jaw was shallower, the symphysis and anterior spout shorter and the ascending portion far lower.

(7) The backbone had a greater number of separate parts; the atlas, as always, was free, the axis was fused with two or three of the following vertebræ; the sixth was free and the seventh fused with the first and second dorsals to form one piece, which was succeeded by two or three separate vertebræ: the other dorsals, except the last one, were united in the dorsal tube, and the lumbo-sacral tube was already complete. Thus, instead of four or five, there were eight or nine distinct parts. None of the tail-vertebræ were fused together.

(8) There was the same disparity in the length of the fore and hind limbs, but the bones were far more slender and armadillo-like; this was especially true of the radius and humerus, the latter having well-developed deltoid and supinator ridges and epicondylar foramen; the ulna was more massive and glyptodont-like. The femur was very much more slender and rounded and the third trochanter was placed higher up the shaft; tibia and fibula were coössified at both ends and resembled those of the Pampean genera, except for their much greater slenderness.

(9) The feet were much as in the latter, but relatively narrower, and the manus had longer claws.

In short, the Santa Cruz †glyptodonts departed much less widely from the armadillos than did the Pliocene and Pleistocene genera, and, to a certain extent, bridged over the gap between the two suborders. Such backward convergence in time is very strong evidence for the community of origin of the two groups.

The †glyptodonts of the more ancient formations, so far as they are known, teach us little concerning the stages of modification in these extraordinary animals, because of their fragmentary condition. The oldest stage in which representatives of the suborder have been detected is the Astraponotus beds, which may be Oligocene or upper Eocene. On the face of the records, therefore, the †glyptodonts had no such antiquity as the armadillos.

* * * * *

It has long been recognized that the Edentata occupy a very isolated position among the placental mammals; their relationships to other orders and their point of departure from the main stem are unsolved problems. The South American fossils have so far thrown little light into these dark places, but they bear very cogent witness to the unity of origin of the five suborders, which were most probably all derived from a single early Eocene or Paleocene group.

In the Paleocene and through most of the Eocene of North America there lived an order of mammals called the †Tæniodontia (or †Ganodonta) which many of the foremost palæontologists regard as an ancestral type of the Edentata, and Dr. Schlosser actually includes them in that order. That the †tæniodonts had certain striking resemblances to the edentates, especially to the †ground-sloths, is not to be denied, but the interpretation of these resemblances is a very complex and difficult question. Unfortunately, no member of the order is known from an even approximately complete skeleton, and therefore a discussion of the matter here would be unprofitable. My own conclusion, however, may be stated, to the effect that the supposed relationship of the †tæniodonts to the edentates is illusory and not real. Definite decision must await the finding of more complete material both of the †tæniodonts and the most ancient South American edentates.