63 



young of the Bradypva tridactylus, prior to the completion of the coalescence of the 

 several bones. 



The five vertebrae composing the sacral part of the pelvis of the Megatherium have 

 been described in § 2, p. 22, treating of the vertebral column to which they belong. 



The ib'ac bones (Plates I. and XXII. 62), as they extend from their place of anchylosis 

 with the sacrum, expand in depth and breadth ; their anterior plane is directed forward, 

 being almost vertical and at right angles with the axis of the spine. Each ilium, after 

 contributing its share to the acetabulum (Plate XXII. fig. 1, a), rapidly contracts to an 

 obtuse point bent downward and outward. The two bones, in a front view, resemble 

 a pair of broad outstretched wings at the sides of the fore part of the sacrum. The 

 anterior surface is slightly concave, but is undulated, with many sharp ridges that have 

 penetrated between the fasciculi of the muscles thereto attached. The 'labrum,' or 

 upper and outer convex border of the ilium, is unusually thick and rugged ; the under 

 concave border is also rugged, but is thin, and in some parts sharp. On the inner side 

 of the acetabulum there is a well-defined, raised and very rough, oblong surface (p) for 

 the insertion of the tendon of a powerful 'psoas' muscle. 



The outer surface of the ilium is slightly concave near the sacrum, and is then convex 

 in the direction of its longest diameter, which is from within outwards ; in other directions 

 it is nearly flat: its surface is much broken by numerous intermuscular ridges. 



The pubis (Plates I., XXII. & XXVII. <u) is very slender where it forms the anterior 

 border of the 'foramen ovale' (Plate XXII. fig. 1, 0), but expands at its extremities, 

 and especially where it coalesces with the ischium to form the produced and pointed 

 'symphysis.' The extent of this symphysis is 10 inches in a straight line. 



The pelvis, as in the Sloths and other members of the Order Bruta, shows the con- 

 version of the ischiadic notches (ib. i) into foramina by the anchylosis of the ischia 

 with the posterior sacral vertebrae (ib. s). Each ischium (Plates I. & XXII. «s), as it 

 extends from its confluence with the sacrum, expands into a broad smooth plate of 

 bone, bent outward and forward, then contracting as it converges inward towards its 

 fellow, to combine with the pubic bones at the symphysis. The hinder border, forming 

 the tuberosity, <&, is thick and rugged ; and two or three perforations here indicate the 

 original line of its separation from the sacrum. The part of the ischium which joins 

 the pubis on the sacral side of the foramen ovale presents on its inner surface the usual 

 oblique channel leading to that foramen. 



Among the existing species of the Order Bruta the Sloths alone resemble the Mega- 

 therium in the expansion of the iliac bones, but this is much less in comparison with 

 the length of the trunk ; the iliac expansion is relatively greater in the Megatherium 

 than in the Elephant, and is associated with a much greater proportionate size of the 

 whole pelvis and of its cavity or channel. The extreme breadth of the pelvis of a large 

 Asiatic Elephant is 3 feet 8 inches, whilst in the Megatherium it is upwards of 5 feet. 



The pelvis which Cuvier was led to suspect, from the defective condition of its fore 

 part in the Madrid skeleton, to be naturally open anteriorly, as in the Myrmeco- 



I 



