64 



phaga didactyla, is closed anteriorly, as in the Sloths, by a 'symphysis pubis' of short 



extent. 



The acetabulum (Plate XXII. fig. 2) presents a full oval shape, with the lower 

 margin bisected by a narrow and deep 'Haversian' groove, which extends, slightly 

 expanding and becoming more shallow, to near the bottom of the cavity ; the outer 

 division of the lower border of the groove is most produced. The large and deep 

 acetabula look downward and a little outward. One diameter of the hemispheroid 

 canty is 8 inches, the other diameter is 7 inches ; it therefore presents a plane surface 

 of 43*9824 square inches, which, multiplied by 15, with the barometer at 30 inches, 

 gives about 660 pounds atmospheric pressure upon the hip-joint of the Megathe- 

 rium. 



The size and strength of the ordinary processes of the pelvis, the breadth of the 

 rough labrum of the iliac bones, and the numerous and well-defined intermuscular 

 crests, indicate the unusual size and vigour of the muscular masses which proceeded 

 from the pelvis in different directions to act upon the trunk and fore limbs and upon the 

 hind limbs and tail. They lead to the conviction that the resistance which demanded 

 such forces for its overcoming must have been of a very different nature and degree 

 from any that now opposes itself to the labours of the existing vegetable feeders 

 when engaged in supplying their daily wants; whence it may be inferred that the 

 exertion of such forces was associated with equally peculiar habits in the megatherioid 

 animals. 



The femur of the Megatherium (Plates I. & XXVII. es, and Plate XXIII. fig. 1) is one 

 of the most massive limb-bones in the Mammalian class ; from its proportions it might 

 rank with the 'flat' instead of the 'long' bones, but that its thickness would rather 

 bring it into the category in which the carpal or tarsal bones are placed according to the 

 old anatomical character derived from shape. 



The head (Plate XXIV. fig. 1) would be a smooth hemisphere, but that the antero- 

 posterior diameter somewhat exceeds the transverse one. Its surface is unimpressed, 

 and its periphery uninterrupted, save by a small entering notch at the middle of its back 

 part (Plate XXIII. fig. 1), into which possibly some thickened band of the capsule, like 

 a ' ligamentum teres,' may have been implanted. The neck of the femur is short and 

 ill-defined ; the upper contour passes from the head to the summit of the great trochanter 

 (ib. t), which is on the same level, in a slightly concave line ; the inner contour of the 

 bone descends with a somewhat deeper concavity from the lower periphery of the head 

 to the shaft. This is flattened from before backward, and presents a slightly oblique 

 twist, the head and the outer condyle being on a plane anterior to the trochanter and 

 inner condyle. The great trochanter presents a broad rugged surface, flattened obliquely 

 from above downward and inward, divided into two somewhat flattened facets by a 

 transverse ridge arching upward. The lower facet contracts as it descends, and is con- 

 tinued into the strong outer ridge which descends to the external condyle. The thick 

 rough border of the rest of the trochanter stands out beyond the contiguous parts of the 



