Chapter VI: Part 6

The astragalus of the Ai (_Bradypus tridactylus_) differs widely from that of either the Megathere, Mylodon(?) or Scelidothere in having a conical cavity on the upper surface, in place of the fibular convexity, in which concavity the distal end of the fibula rotates like a pivot. This mechanism is closely related to the scansorial uses of the inwardly inflected foot of the Sloth.

If the astragalus of an Armadillo[57] were placed side by side with that of the Megathere, it would be very difficult to determine the analogous parts, especially of the upper surface, unless guided by the intermediate structure presented by the Scelidothere. The upper surface of this bone, in the Armadillo, is, however, divided into two transversely convex trochleæ, separated by a much wider transversely concave surface. The fibular trochlea resembles that of the Scelidothere in having its upper and outer facets sloping away at an acute angle, but without meeting at a ridge anteriorly; this surface is not more raised above the tibial trochlea than in the Scelidothere.

The inner trochlea differs from that of the Scelidothere in having a greater relative antero-posterior extent, and in forming, in place of an uniform convex surface, a trochlea similar in structure to that on the outer side. The extent of rough surface on the upper part of the astragalus intervening between the articular surface for the bones of the leg, and that for the scaphoides is extremely small in the Megathere and Mylodon(?); it is relatively greater in the Scelidothere; it is still more extensive in the Armadillo; but is the longest in the Sloth. The anterior extremity of the astragalus which is entirely occupied by the scaphoid articular surface is very peculiar in the Scelidothere (Pl. XXVI. fig. 2.): it presents one convex and two concave facets, which, however, form part of one continuous articular surface: the convex facet forms the internal part of the surface, and presents a rhomboidal form with the long axis vertical. The concave facets (_c_ and _d_) are extended transversely and placed one above the other; they are slightly concave in the transverse, and nearly flat in the vertical directions.

In the Megatherium (fig. 1.) the scaphoid surface of the astragalus is divided only into one concave and one convex portion, both continuous with each other: the concave facet (_c_) corresponds with the upper concavity in the Scelidothere, but is a pretty uniform subcircular depression, fourteen lines in depth: the convex facet, _d_, is continued across the whole breadth of the under part of the scaphoid surface and corresponds with both the inner convex, and lower concave surfaces of the scaphoid articulation in the Scelidothere.

In the Mylodon(?) (Pl. XXVIII. fig. 3.), the articular facet, corresponding with that marked (_c_) in the astragali of the Megathere and Scelidothere, is simply flattened, instead of being concave; the rest of the scaphoid surface corresponds with that in the Megatherium.

In the Armadillo the scaphoid articular surface is undivided and wholly convex: in this part of the astragalus, therefore, we find the Scelidothere deviating from the Armadillo further than does the Megathere; while the Mylodon or Megalonyx(?) most resembles the Armadillo in the configuration of this part of the astragalus.

If we compare the outer surfaces of the astragalus in these quadrupeds, we shall find, however, that the Scelidothere and Armadillo closely agree: the outer facet of the fibular trochleæ, above described, is continued in the Scelidothere (Pl. XXVIII. fig. 2.), upon the fibular side of the astragalus reaching nearly half-way down the posterior part, and down nearly the whole of its anterior.

In the Armadillo, it extends over the whole of the anterior part of the outer side of the astragalus. In both animals the lower boundary of this articular surface describes a strong sigmoid curve.

In the Megatherium (Pl. XXVIII. fig. 1), the corresponding surface for the fibular malleolus on the outer side of the astragalus is formed by a comparatively very small semicircular flattened facet, which by its roughness indicates that the end of the fibula was attached to it by ligamentous substance, and that the synovial bag was not continued upon that surface as in the Scelidothere and Armadillo.

In the Mylodon(?) (Pl. XXVIII. fig. 4), even this rough facet is wanting and the fibular trochlea is bounded by the angle which divides the upper from the outer surface of the astragalus.

Turning now our attention to the under surface of the astragalus, we observe that it presents in the Scelidothere (Pl. XXVI. fig. 6), an irregular quadrate form, having the outer side occupied by an elongated subovate articular facet, _e_, for the calcaneum, bounded externally by a sharp edge, with its long axis and its greatest concavity in the antero-posterior direction, and slightly convex from side to side: a second calcaneal articular surface (_f_) is situated at the inner and anterior angle; it is oblong and nearly flat; is continuous with the inferior concave facet of the scaphoid articulation, but is divided from the convex facet by a groove: the two calcaneal articulations are separated by a deep and rough depression, traversing the under surface of the astragalus diagonally, and increasing in breadth towards the posterior and internal angle. The inner side of the astragalus presents a convex protuberance.

The correspondence between the astragalus of the Scelidothere and Megathere is best seen at the under surface of the bone: in both the two calcaneal articulations are separated by the diagonal depression, and the internal and anterior surface is continuous with the scaphoid articulation. In the Megathere, however, in consequence of the absence of the inferior concavity which characterizes the Scelidothere, the anterior calcaneal facet (_f_) appears as a more direct backward continuation of the scaphoidal surface; but they are divided by a more marked angle than is represented in the figure (fig. 5, Pl. XXVI.). The posterior and outer calcaneal surface in the Megathere (_e_) is broader in proportion to its length, continued further upwards upon the outward surface, is consequently more convex in the transverse direction, and is not bounded externally by so sharp and prominent a ridge as in the Scelidothere. The protuberance from the inner surface of the astragalus is more compressed laterally in the Megathere than in the Scelidothere. The correspondence between the astragali of the Mylodon(?) (Pl. XXVIII. fig. 6) and Megathere in the conformation of the under surface is so close, that the few differences which exist will be sufficiently appreciated by an inspection of the figures.

In the Armadillo the astragalus, in consequence of the greater production of its anterior part, presents more of an angular than a quadrate figure; and the scaphoid articular surface, being proportionally carried forwards, is altogether separated from the anterior calcaneal surface. The posterior and inner calcaneal surface resembles that in the Scelidothere, but is less inclined upwards; and is continuous with the posterior part of the tibial articular surface.

Thus the astragalus in the structure of its two most important articulations, viz. that which receives the superincumbent weight from the leg, and that which transmits it to the heel, presents a closer correspondence in the Scelidothere with that of the Dasypus, than with that of the Megathere or Mylodon.

The ungueal phalanx of the Scelidothere before alluded to, is represented of the natural size in Pl. XXVII. The side view, fig. 3. shows the position of the articular surface on the proximal end, sloping obliquely towards the under surface, and overtopped by an obtuse protuberance, calculated to impede any upward retraction of the claw: the present joint, in fact, illustrates in every particular the argument by which Cuvier established the true affinities of the allied extinct genus Megalonyx.[58]

The present phalanx is, however, less compressed, and less incurved than those of the Megalonyx, which have been hitherto described; but it more resembles in these proportions one of the smaller, and presumed hinder, ungueal phalanges of the Megatherium. The upper and lateral parts of the bone are rounded, and it gradually tapers to the apex, which is broken off. The osseous sheath for the claw is developed only at the under part of the bone: it presents the form of a thick flat plate of bone, with the margin very regularly and obliquely bevelled off, and having a vertical process of bone attached lengthwise to the middle of its under surface. This process must have served for the insertion of a very powerful flexor tendon. The figures of this bone preclude the necessity of any further verbal description.

M. Lund lays most stress upon the argument founded on the inward inflection of the sole of the foot in the Megalonyx, and appeals with greatest confidence to this structure in support of his hypothesis of the scansorial habits of that extinct Edental.[59]

It is quite true that the Quadrumana derive advantage from this position of the foot in climbing trees, and that it is carried to excess in the Sloths, which can only apply the outer edge of the foot to the ground. But we may ask, was the inversion of the sole of the foot actually carried to such an extent in the _Megalonyx_? And, admitting its existence in an inferior degree, is it then conclusive as to the scansorial habits of that species?

M. Lund expressly states that it is produced by a different structure and arrangement of the tarsal bones, from that which exists in the Sloth, but he does not specify the nature of this difference.

If the astragalus, which I have referred with doubt to the _Megalonyx_, do not actually belong to that genus, it is evidently part of a very closely allied species. Now this astragalus, as we have seen, resembles most closely that of the Megatherium; and since we may infer that the calcaneum, scaphoides, and cuboides had a like correspondence, the inclination of the sole of the foot inwards must have been very slight, as I have determined from examination of the structure and co-adaptation of those bones in the incomplete skeleton of the Megatherium in the London College of Surgeons. Such an inclination of the foot may be conceived to have facilitated the bending of the long claws upon the sole, during the ordinary progressive movements of the animal, but it is quite insufficient to justify the conclusion, that it related to an application of the hind-feet for the purposes of climbing.

It is not without interest again to call to mind the deviation of the structure of the astragalus of the Scelidothere from the Megatherioid to the Dasypodoid type of structure. For if the Megatherioid type of structure had really been one suitable to the exigencies of climbing quadrupeds, it might have been expected to have exhibited the scansorial modifications more decidedly, as the species diminished in stature; but as regards the instructive bone of the hind-foot, the modifications of which we have just been considering, this is by no means the case.

DESCRIPTION OF A MUTILATED LOWER JAW OF THE
MEGALONYX JEFFERSONII.

In the preceding section an astralagus was described, which was regarded as belonging possibly to the same Edentate species as the jaw figured and described, p. 69, Pl. XVIII. and XIX., under the name of _Mylodon Darwinii_; but the same correspondence,—that of relative size,—renders it equally possible that this astragalus may belong to the species of _Megalonyx_ to which the lower jaw now under consideration appertains. There could be no doubt, from its structure, that it was the astragalus of a gigantic species of the order _Bruta_, and of the _Megatherioid_ family, and more nearly allied to the Megathere than is the Scelidothere, but sufficiently distinct from both.

The lower jaw, figured in Pl. XXIX., is the only fossil brought home by Mr. Darwin that could be confidently referred to the genus _Megalonyx_; but the form of the tooth in place on the right side of the jaw fully justifies this determination. The jaw itself is deeply and firmly imbedded in the matrix, so that only the upper or alveolar border is visible. The coronoid and condyloid processes are broken away, and the texture of the remaining part of the jaw was too friable, and adhered too firmly to the surrounding matrix to admit of more of its form being ascertained than is figured.

There were four molars on each side of this jaw; the large oblique perforation near the fractured symphysis is the anterior extremity of the wide dental canal. The forms of the alveoli are best preserved in the right ramus: the first is the smallest, and seems to have contained a tooth, of which the transverse section must have been simply elliptical: the second tooth is likewise laterally compressed, but the transverse section is ovate, the great end being turned forwards: the third socket presents a corresponding form, but a larger size: the fourth socket is too much mutilated to allow of a correct opinion being formed as to the shape of the tooth which it once contained. The natural size of the tooth _in situ_, and of the adjoining socket, is given in Pl. XXIX., fig. 2. The difference of form which the jaw of the Megalonyx presents, as compared with that of the Mylodon, especially in the greater recedence of the two horizontal rami from each other, will be appreciated by comparing Pl. XVIII. with Pl. XXIX.

DESCRIPTION OF A FRAGMENT OF THE SKULL AND OF THE TEETH OF THE
MEGATHERIUM CUVIERI.

Notwithstanding the full, accurate, and elaborate accounts of the skeleton of the Megatherium given by Bru,[60] Cuvier,[61] Pander and D’Alton,[62] and Mr. Clift,[63] the fragments of this most gigantic of quadrupeds brought home by Mr. Darwin, possess much interest, and have added, what could hardly have been anticipated, important information as to the dental system, whereby an error in the generic character of the Megatherium has been corrected.

The fragments here alluded to are portions of the skull of three full-grown Megatheres: the most perfect part of which affords a view of the posterior, and of part of the basal surface, which regions of the cranium have not hitherto been elsewhere figured or described, (Pl. XXX.)

The plane of the occipital foramen forms with that of the base of the skull an angle of 140°, the plane of the posterior surface of the skull forms with the basal plane an angle of 68°. The occipital condyles are therefore terminal, or form the most posterior parts of the cranium. The extent of their convex curvature in the antero-posterior direction, which equals that of a semicircle, indicates that the Megatherium possessed considerable freedom and extent of motion of the head. The condyles are not extended in the lateral direction so far as in the Toxodon; their axis is more oblique than in the Glossotherium, and their internal surface is more parallel with the axis of the skull, the foramen magnum not presenting that infundibuliform expansion which is so characteristic of the Glossotherium. The occipital condyles resemble most in form and position those of the Scelidotherium; but in the angle of the occipital plane the Megatherium is intermediate between the Scelidothere and Glossothere. The ex-occipitals terminate laterally and inferiorly, each in a short, but strong obtuse process. The posterior plane of the skull is traversed by a strong arched intermuscular crest, which forms the upper boundary of a pretty deep fossa, which is divided by a median vertical ridge, extending downwards to within an inch of the upper margin of the foramen magnum. A second strong obtuse transversely arched ridge curves over the first, and forms the upper boundary of the posterior or occipital region of the skull: the interspace between the two transverse ridges is very irregular, and indicates the firm implantation of powerful nuchal muscles or ligaments, (Pl. XXX. fig. 1.)

In the configuration and angle of the occipital plane the Megatherium indicates the same general correspondence with the Edentate type, which has been pointed out in the descriptions of the crania of the Glossothere and Scelidothere: and the resemblance to the Scelidothere is not less striking in the small proportional size of the cranium in this quadruped, which surpasses the rest of its class in so great a degree in the colossal proportions of its hinder parts.

Having detected in the base of the skull of the Scelidothere an articular semicircular pit for the head of the styloglossal bone, similar to, but relatively smaller than, that remarkable one in the skull of the Glossothere, it became a matter of interest to determine whether this structure, which does not exist in any of the existing Edentals, should likewise be present in the gigantic type of the Megatherioid family. The result of a careful removal of the matrix from the basal region of one of the cranial fragments of the Megatherium was the detection of this articular cavity, in each temporal bone in the same relative position as in the Glossothere and Scelidothere. The styloid articular cavity is relatively smaller, and shallower, than in the Glossothere, its proportions being much the same as those of the Scelidothere. The cranial or posterior extremity of the stylo-hyoid bone in the Scelidotherium is bent upwards at an obtuse angle (Pl. XXI.), and terminates in an articular ball which rotates in this cavity. The size of this bone, and its mode of articulation, indicates great power and muscularity of tongue in the Megatherioids, and calls to mind the importance of that organ in the Giraffe, which subsists on the same kind of food as that which I have supposed to have supported the Megatherioids, although the general organization of these animals and the mode in which the foliage was brought within reach of the tongue are as opposite as can well be imagined.

The anterior condyloid foramen presents scarcely one-half the absolute size of that of the Glossothere, whence we may infer a correspondingly inferior development of the tongue in the Megathere. The fractured parietes of the cranial cavity of the Megatherium every where exhibit evidences of the great extent of the air-cells or sinuses continued from the nasal cavity: on the basilar aspect of the cranium they extend as far back as the jugular foramina: the whole of the basi-sphenoid being thus excavated, and permeable to air, derived from the sphenoid sinuses, (Pl. XXX. fig. 2.). The vertical diameter of the cranial cavity is four inches, eight lines; its transverse diameter, which is greatest in the posterior third part of the cavity, corresponding with the posterior part of the cerebrum is six inches: from the indications afforded by the remains of the cranial cavity in Mr. Darwin’s specimens, I conclude that the brain of the Megatherium was more depressed, and upon the whole, smaller by nearly one-half than that of the Elephant; but with the cerebellum relatively larger, and situated more posteriorly with relation to the cerebral hemispheres: whence it may be concluded that the Megatherium was a creature of less intelligence, and with the command of fewer resources, or a less varied instinct than the Elephant.

It has been usual to characterize the Megatherium, in conformity with the concurrent descriptions of Bru, Cuvier, and D’Alton, by the dental formula of _molares_ ⁴⁄₄ ⁴⁄₄, i. e. by the presence of four grinding teeth on each side of the upper, as of the lower jaw. It was the agreement of the excellent authorities above cited in this statement, which induced Mr. Clift and myself to regard a single detached tooth, which formed part of the valuable collection of remains of the Megatherium deposited in the Hunterian Museum by Sir Woodbine Parish, as being, from its comparatively small size, the tooth of either a younger individual or of a smaller species of Megatherium. Upon clearing away the matrix from the palatal and alveolar surface of one of the cranial fragments of the Megatherium in Mr. Darwin’s collection, I was gratified by the detection of the crown of a fifth molar, corresponding in size and form with the detached tooth, above alluded to: its small size, and its position have doubtless occasioned its being over-looked in the cranium of the great skeleton at Madrid.

The anterior molar of the upper jaw presents a nearly semicircular transverse section, with the angles rounded off; the three succeeding teeth are four-sided, with the transverse somewhat exceeding the antero-posterior diameter: they are rather longer and larger than the first: the last molar is likewise four-sided, but presents a sudden diminution of diameter, and is relatively broader. The following are the respective dimensions of the upper maxillary teeth.

First Second Third Fourth Fifth
Molar. Molar. Molar. Molar. Molar.
In. Lines. In. Lines. In. Lines. In. Lines. In. Lines.
Length 8 6 9 4 9 4 8 7 5 2
Transverse 1 9 2 4 2 3 2 0 1 4
diameter
Antero-posterior 1 5 2 0 2 0 1 11 0 10
diameter

Besides the differences in size, the upper molars vary as to their curvature: this difference is exhibited in the vertical section of these teeth figured in Pl. XXXI. The convexity of the curve of the first, second and third molars is directed forwards; the fourth is straight, its anterior surface only describing a slight convexity in the vertical direction; the fifth tooth is curved, but in a contrary direction to the others; and the bases of the five molars thus present a general convergence towards a point a little way behind the middle of the series.

The next peculiarity to be noticed in these remarkable teeth is the great length of the pulp-cavity (_d_), the apex of which is parallel with the alveolar margin of the jaw: a transverse fissure is continued from this apex to the middle concavity of the working surface of the tooth, which is thus divided into two parts. Each of these parts consists of three distinct substances,—a central part analogous to the body or bone of the tooth or ‘dentine,’ a peripheral and nearly equally thick layer of _cæmentum_, and an intermediate thinner stratum of a denser substance, which is described in Mr. Clift’s memoir on the Megatherium as ‘enamel,’ and to which substance in the compound teeth of the Elephant, it is analogous both in its relative situation, and relative density to the other constituents.

Microscopic examinations of thin and transparent slices of the tooth of the Megatherium prove, however, that the dense layer separating the internal substance from the cæmentum is not enamel, but presents the same structure as the hard ‘dentine’ or ivory of the generality of Mammalian teeth; and corresponds with the thin cylinder of hard ‘dentine’ in the tooth of the Sloth. No species of the Order _Bruta_ has true enamel entering into the composition of its teeth; but the modifications of structure which the teeth present in the different genera of this order are considerable, and their complexity is not less than that of the enamelled teeth of the Herbivorous Pachyderms and Ruminantia, in consequence of the introduction of a dental substance into their composition corresponding in structure with that of the teeth of the _Myliobates_, _Psammodus_, and other cartilaginous fishes.

The microscopic investigation of the structure of the teeth of the Megatherium was undertaken chiefly with the view of comparing this structure with that of the teeth of the Sloth and Armadillo, and of thus obtaining an insight into the food, and an additional test of the real nature of the disputed affinities of the Megatherium. The central part of the tooth (_c._ Pl. XXXI.) consists of a coarse ivory, like the corresponding part of the tooth of the Sloth. It is traversed throughout by medullary canals ¹⁄₁₅₀₀ of an inch in diameter, which are continued from the pulp-cavity, and proceed, at an angle of 50°, to the plane of the dense ivory, parallel to each other, with a slightly undulating course, having regular interspaces, equal to one and a half diameters of their own areæ, and generally anastomosing in pairs by a loop of which the convexity is turned towards the origin of the tubes of the fine dentine, as if each pair so joined consisted of a continuous reflected canal, (_c._ fig. 1, Pl. XXXII.) The loops are generally formed close to the fine dentine. In a few situations I have observed one of the medullary canals continued across the fine dentine, and anastomosing with the corresponding canals of the cæmentum. The interspaces of the medullary canals of the coarse dentine are principally occupied by calcigerous tubes which have an irregular course, anastomose reticularly, and terminate in very fine cells. The more regular and parallel calcigerous tubes, which constitute the thin layer of hard dentine, are given off from the convexity of the terminal loops of the medullary canals. The course of these tubes (_b._ fig. 1, Pl. XXXII.) is rather more transversely to the axis of the tooth than the medullary canals from which they are continued. They run parallel to each other, but with minute undulations throughout their course, in which they are separated by interspaces equal to one and a half their own diameter. As they approach the cæmentum they divide and sub-divide, and grow more wavy and irregular: their terminal branches take on a bent direction, and form anastomoses, dilate into small cells, and many are seen to become continuous with the radiating fibres or tubes of the cells or corpuscles of the contiguous cæmentum. This substance enters largely into the constitution of the compound tooth of the Megatherium: it is characterized, like the cæmentum of the Elephant’s grinder, by the presence of numerous radiated cells, or purkingian corpuscles, scattered throughout its substance, but may be distinguished by wide medullary canals which traverse it in a direction parallel with each other, and forming a slight angle with the transverse axis of the tooth. These canals are wider than those of the central coarse dentine, their diameter being ¹⁄₁₂₀₀th of an inch; they are separated by interspaces equal to from four to six of their own diameters, divide a few times dichotomously in their course, and finally anastomose in loops, the convexity of which is directed towards, and in most cases is in close contiguity with, the layer of dense dentine.

Fine calcigerous tubes are every where given off at right angles from the medullary canals of the cæmentum, which form a rich reticulation in their interspaces, and a direct continuation between the loops of the medullary canals and the calcigerous tubes of the dense dentine. The cæmentum differs from the coarse dentine in the larger size and wider interspaces of its medullary canals, and by the presence of the bone-corpuscles in their interspaces; but they are brought into organic communication with each other, not only by means of the tubes of the dense dentine, but by occasional continuity of the medullary canals across that substance. The tooth of the Megatherium thus offers an unequivocal example of a course of nutriment from the dentine to the cæmentum, and reciprocally. Retzius observes with respect to the human tooth, that “the fine tubes of the cæmentum enter into immediate communications with the cells and tubes of the dentine (zahnknochen), so that this part can obtain from without the requisite humours after the central pulp has almost ceased to exist.” In the Megatherium, however, those anastomoses have not to perform a vicarious office, since the pulp maintains its full size and functional activity during the whole period of the animal’s existence. It relates to the higher organized condition, and greater degree of vitality of the entire grinder in that extinct species.

The conical cavities (_d._ Pl. XXXI.) attest the size and form of the persistent pulp; the diameter of its base is equal to the part of the crown of the tooth which is formed by the coarse and fine dentine. From the gradual thinning off and final disappearance of these substances as they reach the base of the tooth, I conclude that they were both formed at the expense of the pulp. The fine tubes and cells must have been excavated in its peripheral layer for the reception of the hardening salts of the dense dentine, and the rest converted into the parallel series of medullary canals with their respective systems of calcigerous tubes, in a manner closely analogous to the development of the entire tooth of the Orycteropus. The coarser dentine of the tooth of the Megatherium differs, in fact, from the entire tooth of the _Orycteropus_, only in that the parallel medullary canals and their radiating calcigerous tubes are not separated from the contiguous canals by a distinct layer of cæmentum, and that the medullary canals anastomose at their peripheral extremities. The wide spaces, (_e._ Pl. XXXI.) indicate the thickness of the dental capsule by the ossification of which the exterior stratum of cement was formed. It was not until I knew the true structure of the tooth of the Megatherium, that I could comprehend the mode of its formation. The parallel layers of enamel in the Elephant’s grinder are formed, as is well known, by membranous plates passing from the coronal end of the closed capsule towards the base of the tooth; but a certain extent of enamel can only thus be formed, and when the crown of the grinder has once protruded, and come into use, the enamel cannot be added to. The modification of the structure of the tooth of the Megatherium readily permits the uninterrupted and continuous formation of the dense substance which is analogous to the enamel of the Elephant’s grinder.

With respect to the question of the respective affinities of the Megatherium to the Bradypodoid or Dasypodoid families, the result of this examination of the teeth speaks strongly for its closer relationship with the former group: the Megalonyx, Mylodon, and Scelidotherium, in like manner correspond in the structure of their teeth with the Sloth, and differ from the Armadillo.

If from a similarity of dental structure we may predicate a similarity of food, it may reasonably be conjectured that the leaves and soft succulent sprouts of trees may have been the staple diet of the Megatherioid quadrupeds, as of the existing Sloths. Their enormous claws, I conclude, from the fossorial character of the powerful mechanism by which they were worked, to have been employed, not, as in the Sloths, to carry the animal to the food, but to bring the food within the reach of the animal, by uprooting the trees on which it grew.

In the remains of the Megatherium we have evidence of the frame-work of a quadruped equal to the task of undermining and hauling down the largest members of a tropical forest. In the latter operation it is obvious that the immediate application of the anterior extremities to the trunk of the tree would demand a corresponding fulcrum, to be effectual, and it is the necessity for an adequate basis of support and resistance to such an application of the fore extremities which gives the explanation to the anomalous development of the pelvis, tail, and hinder extremities in the Megatherioid quadrupeds. No wonder, therefore, that their type of structure is so peculiar; for where shall we now find quadrupeds equal, like them, to the habitual task of uprooting trees for food?

DESCRIPTION OF FRAGMENTS OF BONES, AND OF OSSEOUS TESSELATED DERMAL
COVERING OF LARGE EDENTATA.

It is now determined that there once existed in South America, besides the Megatherium, the Megalonyx, and the allied genera described in the preceding pages of the present work, gigantic species of the order _Bruta_ belonging to the Armadillo family, and defended, like the small existing representatives of that family, by a tesselated bony dermal covering. The largest known species of these extinct _Dasypodidæ_ is the _Glyptodon clavipes_, of which the armour and parts of the skeleton have been described by MM. Weiss and D’Alton in the Berlin Transactions for 1827 and 1834: and the generic and specific characters and name, with an account of the dental system, and bones of the extremities, were recorded in the Geological Proceedings for March 1839. It would seem that parts of the same, or a nearly allied gigantic species were described in the same year by M. Lund; under the name of _Hoplophorus_. Of the valuable and interesting discoveries of this able Naturalist I regret that I was not aware until the appearance of a notice of them in the Comptes Rendus for April, 1839.[64] Amongst the fragments of bony tesselated armour in Mr. Darwin’s collection are a few pieces which were found by him, associated with remains of Toxodon and Glossotherium near the Rio Negro in Banda Oriental.[65] These fragments, if we may judge from their thickness, must have belonged to an animal at least as large as the _Glyptodon clavipes_; but the pattern differs in the greater equality of size of the component tesseræ. The thickness of the largest fragment is one inch and a half, the tesseræ vary in diameter from one inch to half an inch, and are separated by grooves about two lines in depth, and two in diameter. The pattern formed by the anastomosis of these grooves is an irregular net-work; the contour of the tesseræ is either unevenly subcircular, hexagonal, pentagonal, or even four-sided; with the sides more or less unequal. In those portions of this armour, where one of the tesseræ exceeds the contiguous ones in size, the imagination may readily conceive it to be the centre of a rosette, around which the smaller ones arrange themselves, but there is no regular system of rosettes, as in the portions of the dermal armour of the Glyptodon figured by Weiss, and those brought to England by Sir Woodbine Parish, in which the central piece is double the size of the marginal ones.

The portions of the tesselated bony dermal covering of a Dasypodoid quadruped, figured in Pl. XXXII. figs. 5 and 4, of the natural size, were discovered folded round the middle and ungueal phalanges, figs. 2 and 3, at Punta Alta, in Bahia Blanca, in an earthy bed interstratified with the conglomerate containing the remains of the fossil Edentals.

In one of these fragments, measuring six inches long by five broad, the tesseræ are arranged in rosettes, and so closely correspond in size and pattern with the bony armour described by M. Lund, as characterizing his species, _Hoplophorus euphractus_, that I feel no hesitation in referring them to that animal. One of the pattern rosettes is figured at fig. 4, together with the thickness of the armour at this part, and the coarse tubulo-cellular structure of the bone. Another portion of dermal armour from the same locality, gives the pattern shown in fig. 5, formed by square or pentagonal tesseræ, arranged in transverse rows; it is certain that this portion of armour belonged to the same animal as the preceding piece; and probably that it constituted part of the transverse dorsal bands of the _Hoplophorus_.

The middle and ungueal phalanx, as well as the portions of armour, are given of the natural size, in Pl. XXXII. The upper and outer surface of the phalanx, is shown in fig. 2. It is smooth and flat; joins the inner surface by a sharp edge, which runs along the upper and inner side of the bone; and passes by a gradual convexity to the under surface; the ridge corresponding with the base of the claw, is feebly developed at the under and lateral parts of the base of the claw. Below the double trochlear joint for the middle phalanx, there are two articular surfaces for two large sesamoid bones.

The middle phalanx corresponds in its small antero-posterior diameter and wedge-shape, with that of the great Glyptodon: but the terminal phalanx is longer and deeper, in proportion to its breadth.

Among the collection of fossils from Punta Alta, in Bahia Blanca, there is an interesting fragment of the head of a gigantic animal of the Edentate order, including the glenoid cavity, and part of the zygomatic process of the left side. The articular surface for the lower jaw, exhibits, in its flatness, extent, and the absence of a posterior ridge, the well-marked characteristics of this part of the Edental structure. It measures two inches four lines in the transverse, and two inches two lines in the antero-posterior diameter. The commencement of the zygomatic process presents a vertical diameter of two inches, and a transverse diameter of eight lines at the thickest part. It is slightly concave at its lower border, and convex above. The small portion of the cranial parietes, which is preserved, exhibits the cellular structure consequent upon the great extension and development of the nasal air-sinuses: this condition of the cranial parietes, has already been noticed in the description of the more perfect skulls of the large extinct Edentata.

NOTICE OF FRAGMENTS OF MOLAR TEETH OF A
MASTODON.

Of the remains of this gigantic extinct Pachyderm, observed by Mr. Darwin at Santa Fé, in Entre Rios, and on the banks of the Tercero, the fragments of the teeth and portions of the skeleton which reached England, are not sufficient to lead to a determination of the species; but sufficiently prove it to have been nearly allied, if not identical, with the _Mastodon angustidens_ of Cuvier, and unquestionably distinct from the _Mastodon giganteum_ of the United States.

NOTICE OF THE REMAINS OF A SPECIES OF
EQUUS,

_Found associated with the extinct Edentals and Toxodon at Punta Alta,
in Bahia Blanca, and with the Mastodon and Toxodon at Santa Fé, in Entre
Rios._

The first of these remains is a superior molar tooth of the right side; it was embedded in the quartz shingle, formed of pebbles strongly cemented together with calcareous matter, which adhered as closely to the tooth in question, as the corresponding matrix did to the associated fossil remains. The tooth was as completely fossilized as the remains of the Mylodon, Megatherium, and Scelidothere; and was so far decomposed, that in the attempt to detach the adherent matrix, it became partially resolved into its component curved lamellæ. Every point of comparison that could be established proved it to differ from the tooth of the common _Equus Caballus_ only in a slight inferiority of size.

The second evidence of the co-existence of the horse with the extinct Mammals of the tertiary epoch of South America reposes on a more perfect tooth, likewise of the upper jaw, from the red argillaceous earth of the Pampas at Bajada de Santa Fé, in the Province of Entre Rios.[66]

This tooth is figured at Pl. XXXII. fig. 13 and 14, from which the anatomist can judge of its close correspondence with a middle molar of the left side of the upper jaw.

This tooth agreed so closely in colour and condition with the remains of the Mastodon and Toxodon, from the same locality, that I have no doubt respecting the contemporaneous existence of the individual horse, of which it once formed part.

This evidence of the former existence of a genus, which, as regards South America, had become extinct, and has a second time been introduced into that Continent, is not one of the least interesting fruits of Mr. Darwin’s palæontological discoveries.

DESCRIPTION OF REMAINS OF RODENTIA, INCLUDING THE JAWS AND TEETH OF AN
EXTINCT SPECIES OF
CTENOMYS.

The fragment of the upper jaw, figured in Pl. XXXII. fig. 6, exhibits the first and second molar _in situ_, and the socket of the third and fourth molar, of a Rodent, which by the form and number of the upper maxillary teeth is referable to the genus Ctenomys. The molars are a little larger, the longitudinal groove on their external surface is somewhat deeper, and the last molar is relatively wider than in the existing subterraneous species,—the Tucutucu (_Ctenomys Brasiliensis_, Bl.), of whose habits so interesting an account is given in the description of the Mammalia of the present Collection (No. IV. p. 79). The form of the grinding surface of the first and second upper molar is shown below the fig. 6, and three views of the second grinder are given at figs. 7, 8, and 9. The fragment of the lower jaw of the same fossil Rodent is figured at fig. 10 and 11. The long anterior incisor is relatively narrower than in the _Ctenomys Brasiliensis_. I have not had the means of comparing this fossil with the _Ctenomys Magellanicus_; but since it is probable that the _Ct. Magellanicus_ may not be specifically different from the _Ct. Brasiliensis_, it may be concluded that the present fossil is equally distinct from both.

The portion of the right hind-foot of the Rodent figured at fig. 12, includes the calcaneum, astragalus, cuboides, external and middle cuneiform bones, and the metatarsals and proximal phalanges of the toes corresponding with the three middle toes of five-toed quadrupeds. The metatarsals are chiefly remarkable for the well-developed double-trochlear articular surface, and intermediate ridge. These remains, as well as the jaws and teeth of the Ctenomys, were discovered at Monte Hermoso in Bahia Blanca.

In the same reddish earthy stratum of that locality, Mr. Darwin discovered the decomposed molar of a Rodent, equalling in size, and closely resembling in the disposition of its oblique component laminæ, the hinder molar of the Capybara (_Hydrochærus_). The fossil differs, however, in the greater relative breadth of the component laminæ.

I have, lastly, to notice the head of a femur, and some fragments of pelvic bones from the same formation which bear the same proportion to the tooth above alluded to, as subsists between the teeth and bones of the Capybara, and which are sufficient to prove that there once has existed in South America a species of the family _Caviidæ_, as large as the present _Capybara_, but now apparently extinct.

This fact, together with the greater part of those which have been recorded in the foregoing pages of the present work, establishes the correspondence, in regard to the characteristic type, which exists between the present and extinct animals of the South American Continent: we have abundant evidence likewise of the greater number of generic and specific modifications of these fundamental types which the animals of a former epoch exhibited, and also of the vastly superior size which some of the species attained.

At the same time it has been shewn that some of the present laws of the geographical distribution of animals would not have been applicable to South America, at the period when the Megatherioids, Toxodon, and Macrauchenia existed: since the Horse, and according to M. Lund, the Antelope and the Hyæna, were then associated with those more strictly South American forms. The Horse, which, as regards the American continent, had once become extinct, has again been introduced, and now ranges in countless troops over the pampas and savannahs of the new world. If the small Opossums of South America had been in like manner imported into Europe, and were now established like the Squirrels and Dormice in the forests of France, an analogous case would exist to that of the Horse in South America, as the fossil Didelphys of Montmartre proves.

With respect to the geological contemporaneity of the fossils collected by him, Mr. Darwin subjoins the following observations:—

“The remains of the following animals were embedded together at Punta
Alta in Bahia Blanca:—The _Megatherium Cuvierii_, _Megalonyx
Jeffersonii_, _Mylodon Darwinii_, _Scelidotherium leptocephalum_,
_Toxodon Platensis (?)_ a Horse and a small Dasypodoid quadruped,
mentioned p. 107; at St. Fé in Entre Rios, a Horse, a Mastodon,
_Toxodon Platensis_, and some large animal with a tesselated osseous
dermal covering; on the banks of the Tercero the Mastodon, Toxodon,
and, according to the Jesuit Falkner, some animal with the same kind
of covering; near the Rio Negro in Banda Oriental, the _Toxodon
Platensis_, Glossotherium, and some animal with the same kind of
covering. To these two latter animals the _Glyptodon clavipes_,
described by Mr. Owen in the Geological Transactions, may, from the
locality where it was discovered, and from the similarity of the
deposit which covers the greater part of Banda Oriental, almost
certainly be added, as having been contemporaneous. From nearly the
same reasons, it is probable that the Rodents found at Monte Hermoso
in Bahia Blanca, co-existed with the several gigantic mammifers from
Punta Alta. I have, also, shown in the Introduction, that the
_Macrauchenia Patachonica_, must have been coeval, or nearly so, with
the last mentioned animals. Although we have no evidence of the
geological age of the deposits in some of the localities just
specified, yet from the presence of the same fossil mammifers in
others, of the age of which we have fair means of judging, (in
relation to the usual standard of comparison, of the amount of change
in the specific forms of the invertebrate inhabitants of the sea,) we
may safely infer that _most_ of the animals described in this volume,
and likewise the Glyptodon, were strictly contemporaneous, and that
_all_ lived at about the same very recent period in the earth’s
history. Moreover, as some of the fossil animals, discovered in such
extraordinary numbers by M. Lund in the caves of Brazil, are identical
or closely related with some of those, which lately lived together in
La Plata and Patagonia, a certain degree of light is thus thrown on
the antiquity of the ancient Fauna of Brazil, which otherwise would
have been left involved in complete darkness.”

LONDON:

PRINTED BY STEWART AND MURRAY,

OLD BAILEY.

Pl. I.

_G. Scharf del et lithog._ _Printed by C. Hullmandel._

_Base of the Skull of Taxodon Platensis._

_Nat. Size._

_Published by Smith, Elder & Cº͈ 65, Cornhill, London._
]

Pl. II.

_G. Scharf del et lithog._ _Printed by C. Hullmandel._

_Side View of the Skull of Taxodon._

_One-third the Natural Size._

_Published by Smith, Elder & Cº͈ 65, Cornhill, London._
]

Pl. III.

_G. Scharf del et lithog._ _Printed by C. Hullmandel._

_Top View of the Skull of the Taxodon._

_One-third the Nat. Size._

_Published by Smith, Elder & Cº͈ 65, Cornhill, London._
]

Pl. IV.

_G. Scharf del et lithog._ _Printed by C. Hullmandel._

_Taxodon Platensis._

_Published by Smith, Elder & Cº͈ 65, Cornhill, London._
]

Pl. V.

_G. Scharf del et lithog._ _Printed by C. Hullmandel._

_Fragments of the lower Jaw and Teeth of a Taxodon._

_Nat. Size._

_Published by Smith, Elder & Cº͈ 65, Cornhill, London._
]

Pl. VI.

_G. Scharf del et lithog._ _Printed by C. Hullmandel._

_Cervical Vertebræ of Macrauchenia._

_Published by Smith, Elder & Cº͈ 65, Cornhill, London._
]

Pl. VII.

_G. Scharf del et lithog._ _Printed by C. Hullmandel._

_Cervical Vertebræ of
1, 2. Macrauchenia 3,4 Auchenia._

_Published by Smith, Elder & Cº͈ 65, Cornhill, London._
]

Pl. VIII.

_G. Scharf del et lithog._

_Lumbar Vertebræ, Macrauchenia._

_Fig. 1. Posterior View of last lumbar. Fig: 2,3&4. Fourth lumbar
Vertebra._

_Nat. Size._

_Published by Smith, Elder & Cº͈ 65, Cornhill, London._
]

Pl. IX.

_Lithog. from Nature by G. Scharf._

_Macrauchenia._

_Fig. 1_2. Scapula. Fig. 3. Femur._

_Published by Smith, Elder & Cº͈ 65, Cornhill._
]

Pl. X.

_G. Scharf del et lithog._

_Proximal Extremity of anchylosed Ulna and Radius Macrauchenia._

_⅔ Nat. Size._

_London, Published by Smith, Elder & Cº͈ 65, Cornhill._
]

Pl. XI.

_G. Scharf del et lithog._ _Printed by C. Hullmandel._

_Bones of the right fore-foot, Macrauchenia._

_Fig. 1, ⅔, 2_9, Nat. Size._

_Published by Smith, Elder & Cº͈ 65, Cornhill._
]

Pl. XII.

_Lithog. from Nature by G. Scharf._ _Printed by C. Hullmandel._

_⅔ Nat. Size._

_Right Femur. Macrauchenia._

_Published by Smith, Elder & Cº͈ 65, Cornhill._
]

Pl. XIII.

_Lithog. from Nature by G. Scharf._ _Printed by C. Hullmandel._

_Macrauchenia._

_Right Tibia and Fibula.—Fig. 2_4. ⅔ Nat. Size._

_Published by Smith, Elder & Cº͈ 65, Cornhill._
]

Pl. XIV.

_Lithog. from Nature by G. Scharf._

_Right Astragalus., Macrauchenia._

_Nat. Size._

_Published by Smith, Elder & Cº͈ 65, Cornhill._
]

Pl. XV

_Lithog. from Nature by G. Scharf._ _Printed by C. Hullmandel._

_Macrauchenia._

_Fig. 1. Metatarsal. 2_5. Metacarpals. Nat. Size._

_Published by Smith, Elder & Cº͈ 65, Cornhill._
]

Pl. XVI.

_Lithog. from Nature by G. Scharf._ _Printed by C. Hullmandel._

_Fragment of the Cranium of the Glossotherium_

_½ Nat. Size._
]

Pl. XVII.

_Fig. 3. 4. Laurillard del. Fig. 5. G. Scharf del et lithog._ _Printed
by C. Hullmandel._

_1. Megalonyx Jeffersoni. 2. Meg. laqueatus. 3. 4. Mylodon Harlani. 5.
Myl. Darwinii._

_Published by Smith, Elder & Cº͈ 65, Cornhill._
]

Pl. XVIII

_G. Scharf del et lithog._ _Printed by C. Hullmandel._

_Mylodon. ⁵⁄₉ Nat Size._

]

Pl. XIX

_G. Scharf del et lithog._ _Printed by C. Hullmandel._

_Mylodon._

_Fig 1. ⁵⁄₉ Nat. Size. Fig. 2.3.4. Nat. Size._
]

Pl. XX.

_Lithog. from Nature by G. Scharf._ _Printed by C. Hullmandel._

_Scelidotherium._

_⅔ Nat. Size._

_Published by Smith, Elder & Cº͈ 65, Cornhill._
]

Pl. XXI.

_G. Scharf del et lithog._ _Printed by C. Hullmandel._

_Scelidotherium._

_Fig 1 & 2 ⅔ Nat. Size. Fig 3_5. Nat. Size._

_Published by Smith, Elder & Cº͈ 65, Cornhill, London._
]

Pl. XXII.

_Lithog. from Nature by G. Scharf._ _Printed by C. Hullmandel._

_Scelidotherium._

_Published by Smith, Elder & Cº͈ 65, Cornhill, London._
]

Pl. XXIII.

_Lithog. from Nature by G. Scharf._

_Cranial Cavity and Dentition of Scelidotherium._

_Nat. Size._

_Published by Smith, Elder & Cº͈ 65, Cornhill._
]

Pl. XXIV.

_Lithog. from Nature by G. Scharf._

_Cervical and Anterior dorsal Vertebræ_

_Fig: 1. Scelidothere. Fig: 2. Orycterope. Fig: 3. Armadillo. Fig:4.
Great Ant-eater._

_One-third Nat. Size._

_Published by Smith, Elder & Cº͈ 65, Cornhill._
]

Pl. XXV.

_Lithog. from Nature by G. Scharf._ _Printed by C. Hullmandel._

_Scelidotherium ⅓ Nat. Size._

_Published by Smith, Elder & Cº͈ 65, Cornhill._
]

Pl. XXVI.

_Printed by C. Hullmandel._

_Left Astragalus_

_Fig. 1.3.5. Megatherium: ⅓ Nat. Size. 2.4.6 Scelidotherium. ⅔ Nat.
Size._

_Published by Smith, Elder & Cº͈ 65, Cornhill._
]

Pl. XXVII.

_Lithog. from Nature by G. Scharf._

_Scelidotherium._

_Fig. 1.2. ⅔ Nat. Size. 3.4.5 Nat. Size._

_Published by Smith, Elder & Cº͈ 65, Cornhill._
]

Pl. XXVIII.

_Lithog. from Nature by G. Scharf._ _Printed by C. Hullmandel._

_Left Astragalus._

_Fig. 1. Megatherium. ⅓ Nat. Size. Fig: 2. Scelidotherium. ⅔ Nat.
Size. Fig. 3–6, Mylodon.? ⅔ Nat. Size._
]

Pl. XXIX.

_Lithog. from Nature by G. Scharf._ _Printed by C. Hullmandel._

_Lower Jaw of Megalonyx._

_Fig. 1. ⅔. Fig. 2. Nat. Size._
]

Pl. XXX.

_Lithog. from Nature by G. Scharf._ _Printed by C. Hullmandel._

_Megatherium ½ Nat. Size_
]

Pl. XXXI.

_Lithog. from Nature by G. Scharf._ _Printed by C. Hullmandel._

_Section of the superior maxillary teeth_,

_Megatherium._

_¾ Nat. Size._
]

Pl. XXXII.

_Lithog. from Nature by G. Scharf._ _Printed by C. Hullmandel._

_1. Megatherium. 2_5. Hoplophorus. 6_12. Ctenomys. 13_14. Equus._
]

-----

Footnote 1:

See Ossemens Fossiles, Ed. iv. tom. ii. p. 368. Pl. 27. fig 1. 12.

Footnote 2:

_Ibid._ p. 370. Pl. 27. fig. 5.

Footnote 3:

_Ibid._ p. 347, 367.

Footnote 4:

_Ibid._ p. 337. Pl. 26. fig. 7.

Footnote 5:

Philosophical Transactions, vol. lviii. p. 34. (1768.)

Footnote 6:

Bridgewater Treatise, p. 139.

Footnote 7:

Geological Transactions, vol. iii. p. 437. pl. 44, 45, 46.

Footnote 8:

Quoted by Cuvier, Ossem. Foss. Ed. iv. tom. ii. p. 351.

Footnote 9:

Τοξον, arcus; οδους, dens.

Footnote 10:

Mem. de l’Acad. des Sciences de Paris, 1764, p. 568.

Footnote 11:

True fangs exist only in teeth of temporary growth, they may be one or
more in number, but always diminish in size as they recede from the
crown of the tooth, and are either solid, or with a very small canal.

Footnote 12:

This was written before an examination of the fragment of a lower jaw,
forming part of Mr. Darwin’s collection of Fossil Remains, had led me
to suspect that it was referrible to the genus Toxodon; should this
suspicion prove correct, the four unequal incisors of the upper jaw
are opposed to six equal sized ones in the lower.

Footnote 13:

I have ascertained that this elastic ligament exists in the neck of
the Dugong.

Footnote 14:

The German Translator (See _Frorieps Notizen_., 1837, p. 119) of the
abstract of my description of the Toxodon, published in the
Proceedings of the Geological Society, asks, what is the _Mutica_
(misprinted _Muticata_), of Linnæus? The term is quoted from the
Systema Naturæ, Ed. xii. p. 24. Linnæus first divides Mammalia into
three groups, according to modifications of the locomotive organs,
viz. _Unguiculata_, _Ungulata_, _Mutica_, and subdivides these,
according to modifications of the dentary organs, into the orders,
_Bruat_, _Glires_, _Primates_, &c.

Footnote 15:

Besides the relation to _food requiring much comminution_, which teeth
with persistent pulps bear, they are also connected with the
_longevity of the individual_. The term of life in a herbivorous
animal, with grinders of temporary growth, is, of necessity, dependent
on the duration of these essential aids to nutrition; thus, a sheep
generally wears down its grinders in twelve years, and its natural
term of life is consequently limited to about that period.

Footnote 16:

Μακρος _longus_, αυχην _cervix_: from the latter word Illiger derived
_Auchenia_, his generic name of the Llama, Vicugna, &c.

Footnote 17:

In the seventh cervical vertebra of the Camel, as in many other
Mammalia, there is no perforation in any part for the vertebral
arteries. In a Vicugna, I find the same structure; but in a Llama, the
side of the body of the seventh cervical vertebra is perforated
longitudinally on the right side. In the Camel, the vertebral arteries
pierce the sixth cervical vertebra, immediately below the superior
transverse processes, and pass obliquely to the anterior aperture of
the cervical canal, where they emerge beneath the anterior oblique
processes, and then enter the spinal canal of the fifth cervical
vertebra, as described in the text.

Footnote 18:

Cuvier, Ossemens Fossiles, iii. p. 238.

Footnote 19:

Loc. cit. p. 234.

Footnote 20:

Loc. cit. p. 235.

Footnote 21:

Loc. cit. p. 237.

Footnote 22:

Loc. cit. p. 232.

Footnote 23:

See Ossem. Fossiles, Pl. XX. fig. 3.

Footnote 24:

Loc. cit. Pl. XXII. fig. 6.