120 FOSSIL MAMMALIA. 



Lower Jaw. 



The lower jaw (PI. 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 extra- 

 ordinary and strong process (b) descending from the zygo- 

 matic arch in the Megatherium, as well as in the Sloths, 

 seems intended to support the unusual weight of the lower 

 jaw consequent upon the pecuhar form of the molar teeth. 



Bones of the Trunk. 



The vertebrae 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, compa- 

 ratively light, and without tusks. The dorsal portion of the 

 vertebral column is of moderate size, but there is an enlarge- 

 ment of the vertebras of the loins, corresponding with the ex- 

 traordinary bulk of the pelvis and hind legs; the summits of 

 the spinous processes, (e,) are flattened like those in the Ar- 

 madillo, 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 vertebrce of enormous mag- 

 nitude, (PI. 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 



