156 FOSSIL MAMMALIA. 
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, presents a further 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 obliquity faci- 
litates the movement of the hind leg ; but in 
the Megatherium it is set perpendicularly down- 
wards, 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 diminished 
capability of rapid motion.^ 
From the enormous width of the pelvis, it 
* There is also a further peculiarity for the increase of strength 
in the manner in which that part, which, in most other animals, 
is an open space, called the ischiatic notch (PL 5, Fig. 2 c), is 
nearly closed with solid bone by the union of the spines of the 
ischia with the elongated transverse processes of the sacral verte- 
brae, (a). 
Further evidence of the enormous size and power in the mus- 
cles 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 mar- 
row : this cavity being about four inches in diameter, the spinal 
marrow must have been a foot in circumference. The extraor- 
dinary magnitude also of the nerves which proceeded from it to 
supply the leg, is indicated by the jnodigious size of the sacral 
foramina. 
