150 FOSSIL MAMMALIA. 
for the continual growth and firm fixture of the 
long and vertical molar teeth ; the extraordinary 
and strong process (b) descending from the zy- 
gomatic arch in the Megatherium, as well as in 
the Sloths, seems intended to support the un- 
usual weight of the lower jaw consequent upon 
the peculiar form of the molar teeth. 
JBones 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, comparatively 
light, and without tusks. The dorsal portion 
of the vertebral column is of moderate size, but 
there is an enlargement of the vertebrae 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, (PI. 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 vertebrae of 
enormous magnitude, (PI. (J, Fig. 2), the body of 
the largest being seven inches in diameter, and 
