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
