124 FOSSIL MAMMALIA. 



Posterior Extremities. 



The pelvis of the Megatherium (PI. 5, Fig. 2. p.) is of 

 vast solidity and expanse; and the enormous bones of the 

 ileum (r) are set nearly at right angles to the spine of the 

 back, and at their outer margin, or crest, are more than five 

 feet asunder, very much exceeding the diameter across the 

 haunches of the targest elephant : the crest of the ileum, (s,) 

 is much flattened, as if by the pressure of the armour. This 

 enormous size of the pelvis would be disproportionate and 

 inconvenient to an animal of ordinary stature and functions; 

 but was probably attended with much advantage to the 

 Megatherium, in relation to its habit of standing great part 

 of its time on three legs, whilst the fourth was occupied in 

 digging. 



The pelvis being thus, unusually wide and heavy, pre- 

 sents a farther deviation from other animals, as to the place 

 and direction of the acetabulum, or socket which articulates 

 with the head of the thigh-bone (u.) This cavity, in other 

 animals, is usually set more or less obliquely outwards, and 

 by this obliquitj^ facilitates the movement of the hind-legj 

 but in the Megatherium it is set perpendicularly downwards, 

 over the head of the femur, and is also nearer than usual to 

 the spine; deriving from this position increase of strength 

 for supporting vertical pressure, but attended with a dimi- 

 nished capability of rapid motion.* 



* There is also a farther peculiarity for the increase of strcngtli in the man- 

 ner in wliich that part, which, in most other animals, is an open space, called 

 the ischiatic notch (Pi. 5, Fig. 2 c.) is nearly closed with solid bone by th# 

 union of the spines of the ischia with the elongated transverse processes of 

 the sacral vertebrae, (a.) 



Farther evidence of the enormous size and power in the muscles of the 

 thigh and leg is afforded by the magnitude of the cavity in the sacrum, (PI, 

 5. d,) for the passage of the spinal marrow: this cavity being about four 

 inches in diameter, the spinal marrow must have been a foot in circumfe- 

 rence. The extraordinary magnitude also of the nerves which proceeded 

 from it to supply the leg, is indicated by the prodigious size of the sacrai 

 foramina. 



