Page:Geology and Mineralogy considered with reference to Natural Theology, 1837, volume 1.djvu/128

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The pelvis of the Megatherium (Pl. 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 live feet asunder, very much exceeding the diameter across the haunches of the largest 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, 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 facilitates the movement of the hind-leg; 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 diminished capability of rapid motion.