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XXXII. On the Megatheeium (Megatherium Americanum, Cuviee and Blumenbach). 
Pait V.- Sones of the Posteviov ExtveTnities. Ey ProfessoT Owen, P.R.S., 
Superintendent of the Natural History Departments in the British Museum. 
Eeceived May 6, — Eead May 8, 1851. 
In the description of the bones of the hind limbs of the Megatherium I commence with 
the ilium, as being the homotype or correlative of the scapula in the fore limb. The 
ischium, which is the homotype of the coracoid, is confluent with the ilium, as the 
coracoid is with the scapula : the pubis, which is the homotype of the clavicle, is con- 
fluent with both the ilium and ischium. All the three bones on both sides become 
confluent with the sacrum, and form therewith a single compound bone— the pelvis— 
which is the largest known amongst recent or extinct terrestrial Mammals, and gives the 
most striking feature to the skeleton of the present gigantic extinct Sloth. 
As a like progressive ossiflcation brings to pass a similar state of the pelvis in the 
small existing Sloths, the limits of the primitive bony constituents of that of the Mega- 
therium can only be determined by analogy with the pelvis of its modern congeners, 
studied in individuals at a period of immaturity. This I have had the opportunity of 
doing in the young of the Bradypus tridactylus, prior to the completion of the coales- 
cence of the several bones. 
The five vertebrae composing the sacral part of the pelvis of the Megatherium have 
been described in a former memoir*, treating of the vertebral column to which they 
belong. 
The iliac bones (Plate XXXVII., 62 , and Philosophical Transactions, 1865, Plate 
XXIII.), 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 («), 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, 
* “ On the Megatherium,” Philosophical Transactions, Part II. 1851, p. 378. 
MDCCCLIX. 5 0 
