form to another is so gradual, and the functions of one species receive such ample and obvious illustrations from those of the species adjacent to it, that we are rarely at a loss, to see the final cause of almost every arrangement that is presented to the anatomist. This is more especially the case with respect to the skeleton, which forms the foundation of all the other mechanisms within the body, and is of the highest importance in the history of fossil animals, of which we rarely find any other remains besides the bones, and teeth, and the scaly or osseous integuments. I select the Megatherium, because it affords an example of most extraordinary deviations, and of egregious apparent monstrosity; viz. the case of a gigantic animal exceeding the largest Rhinoceros in bulk, and to which the nearest approximations that occur in the living world, are found in the not less anomalous genera of Sloth, Armadillo, and Chlamyphorus; the former adapted to the peculiar habit of residing upon trees; the two latter constructed with unusual adaptations to the habit of burrowing in search of their food and shelter in sand; and all limited in their geographical distribution, nearly to the same regions of America that were once the residence of the Megatherium.

I shall not here enter on the unsettled questions as to the precise age of the deposites in which the Megatherium is found, or the causes by which it has been extirpated; my object is to show that the apparent incongruities of all its parts, are in reality systems of wise and well contrived adap-. tation to a peculiar mode of life. I proceed therefore to consider, in the order in which, they are described by Cuvier, the most important organs of the Megatherium, beginning with the head, and from thence advancing to the trunk and extremities.

Head.

The bones of the head (Pl. 5, Fig. 1. a.) most nearly re

semble those of a Sloth. The long and broad bone, (b,) descending the cheek from the zygomatic arch, connects it more nearly with the Ai than with any other animal: this extraordinary bone must have been auxiliary to the power of muscles, acting with more than usual advantage, in giving motion to the lower jaw (d.)

The anterior part of the muzzle (c) is so strong and substantial, and so perforated with holes for the passage of nerves and vessels, that we may be sure it supported some organ of considerable size: a long trunk was needless to an animal possessing so long a neck; the organ was probably a snout, something like that of the Tapir, sufficiently elongated to gather up roots from the ground. The septum of the nostrils also being strong and bony, gives farther indication of the presence of a powerful organ appended to the nose; such an apparatus would have afforded compensation for the absence of incisor teeth and tusks. Having no incisors, the Megatherium could not have lived on grass. The structure of the molar teeth (Pl. 5, Fig. 6—11, and Pl. 6, No. 1, shows that it was not carnivorous.

The composition of a single molar tooth resembles that of one, of the many denticules, that are united in the compound molar of the Elephant; and affords an admirable exemplification of the method employed by Nature, whereby three substances, of unequal density, viz. ivory, enamel, and crusta petrosa, or cœmentum, are united in the construction of the teeth of graminivorous animals. The teeth are about seven inches long, and nearly of a prismatic form (Pl. 5, Fig. 7. 8.) the grinding surfaces (Pl. 5. Fig. 9. a. b. c. and Pl. 6, Z. a. b. c.) exhibit a peculiar and beautiful contrivance for maintaining two cutting wedge-shaped salient edges, in good working condition during the whole existence of the tooth; being, as I before stated, a modification of the contrivance employed in the molars of the Elephant, and other herbivora. The same principle is applied by tool-makers for the purpose of maintaining a sharp edge in axes, scythes, bill-hooks,

&c. An axe, or bill-hook, is not made entirely of steel, but of one thin plate of steel, inserted between two plates of softer iron, and so enclosed that the steel projects beyond the iron, along the entire line of the cutting edge of the instrument. A double advantage results from this contrivance; first, the instrument is less liable to fracture than if it were entirely made of the more brittle material of steel; and secondly, the cutting edge is more easily kept sharp by grinding down a portion of exterior soft iron, than if the entire mass were of hard steel. By a similar contrivance, two cutting edges are produced on the crown of the molar teeth of the Megatherium. (See Pl. 6, W. X. Y. Z. and Pl. 5, Figs. 6-10.*)

Pl. 6, W. X. represents the manner in which each lower tooth was opposed to the tooth above it, so that the hard enamel of the one should come in contact only with the softer materials of the other; viz. the edges of the plates of

*The outside of the tooth, like that of an axe, is made of a comparatively soft material, viz. the crusta petrosa, (a a,) enclosing a plate of enamel, (b b,) which is the hardest substance, or steel of the tooth. This enamel passes twice across the grinding surface, (z,) and forms the cutting edges of two parellel wedges, Y. b. b.: a longitudinal-section of these wedges is seen, Pl. 6. v. w. X. Y. Within the enamel, (b. b,) is a central mass of ivory, (c,) which, like the external crust, (a) is softer than the enamel. A tooth, thus constructed of materials of unequal density, would have its softer parts, (a c,) worn down more readily than the harder plates of enamel, (b b.)

We find a farther nicety of mechanical contrivance, for producing and maintaining two transverse wedges upon the surface of each tooth, in the relative adjustment of the thickness, of the lateral and transverse portions of the plate of enamel, which is interposed between the external crust, (a,) and the central ivory, (c.) Had this enamel been of uniform thickness all round the central ivory, the tooth would have worn down equally to a horizontal surface. In the crown of the tooth, Pl. 6, Z. the plate of enamel is seen to be thin on the two sides of the tooth, whilst the transverse portions of the same plate, (b. b.) are comparatively thick and strong. Hence the weaker lateral portions of thin enamel wear away more rapidly, than the thicker and stronger transverse portions, (b b,) and do not prevent the excavation of the furrow across the surface of the ivory, c.

enamel, (b) rubbing upon the ivory, (c;) and the enamel, (b',) upon the crusta petrosa, (a,) of the two teeth opposite to it. Hence the act of mastication formed and perpetually maintained a series of wedges, locking into each other like the alternate ridges on the rollers of a crushing-mill; and the mouth of the Megatherium became an engine of prodigious power, in which thirty-two such wedges formed the grinding surfaces of sixteen molar teeth; each from seven to nine inches long, and having the greater part of this length fixed firmly in a socket of great depth.

As the surfaces of these teeth must have worn away with much rapidity, a provision, unusual in molar teeth, and similar to that in the incisor teeth of the Beaver and other Rodentia,* supplied the loss that was continually going on at the crown, by the constant addition of new matter at the root, which for this purpose remained hollow, and filled with pulp during the whole life of the animal.†

It is scarcely possible to find any apparatus in the mechanism of dentition, which constitutes a more powerful engine for masticating roots, than was formed by these teeth of the Megatherium; accompanied also by a property, which is the perfection of all machinery, namely, that of maintaining itself perpetually in perfect order, by the act of performing its work.

* The incisors of the Beaver, and other Rodentia, and tusks of the Hog and Hippopotamus, which require only an external cutting edge, and not a grinding surface, are constructed on the same principle as the cutting edge of a chissel or an adze; viz. a plate of hard enamel is applied to the outer surface only, of the ivory of these teeth, in the same manner as the outer cutting edge of the chissel and adze is faced with a plate of steel, welded against an inner plate of softer iron. A tooth thus constructed maintains its cutting edge of enamel continually sharp, by the act of working against the similarly constructed extremity of the tooth opposed to it.

† Pl. 5, Fig. 11, represents the section of the cavity containing this pulp.

Lower Jaw.

The lower jaw (Pl. 5, 1. d.) is very large and weighty in proportion to the rest of the head; the object of this size being to afford deep sockets for the continual growth and firm fixture of the long and vertical molar teeth; the extraordinary and strong process (b) descending from the zygomatic arch in the Megatherium, as well as in the Sloths, seems intended to support the unusual weight of the lower jaw consequent upon the peculiar form of the molar teeth.

Bones of the Trunk.

The vertebræ of the neck, though strong, are small in comparison with those towards the opposite extremity of the body; being duly proportioned to the size of a head, comparatively light, and without tusks. The dorsal portion of the vertebral column is of moderate size, but there is an enlargement of the vertebræ of the loins, corresponding with the extraordinary bulk of the pelvis and hind legs; the summits of the spinous processes, (e,) are flattened like those in the Armadillo, as if by the pressure of a cuirass.

The sacral bone, (Pl. 5, Fig. 2, a,) is united to the pelvis, (p,) in a manner peculiar to itself, and calculated to produce extraordinary strength; its processes indicate the existence of very powerful muscles for the movement of the tail. The tail was long, and composed of vertebræ of enormous magnitude, (Pl. 6, Fig. 2,) the body of the largest being seven inches in diameter, and the horizontal distance between the extremities of the two transverse processes, being twenty inches. If to this we add the thickness of the muscles and tendons, and of the shelly integument, the diameter of the tail, at its largest end, must have been at least two feet; and its circumference, supposing it to be nearly circular like the tail of the Armadillo, about six feet. These vast dimensions