67 



The spinous processes of all the anchylosed vertebrse form one continuous 

 vertical bony crest (g, g), which rises, in the first lumbar, to nearly three inches 

 in height, and so continues with very little diminution to the end of the sacrum. 

 This remarkable crest is about four lines in thickness, and slightly expands at 

 the upper margin : it describes a single and moderate convex curve. The ver- 

 tical diameter of the middle of the sacrum, including this ridge, is nine inches, 

 which gives the height of the sixth sacral vertebra, counting from the beginning 

 of the anchylosis. 



The breadth of the last lumbar vertebra, where its transverse processes join 

 the iha, is one foot. The breadth of the sacrum between the ischiadic foramina, 

 is seven inches and a half : the breadth of the posterior end of the sacrum where 

 the two foramina indicate its junction with the ischia, is eleven inches. 



Os innominatum. — The ilium, ischium and pubis, the analogues respectively 

 of the scapula, coracoid and clavicle, are so completely and extensively united 

 with the sacrum, that the great whole which they constitute, and which in the 

 Megatherium is the largest single bone in any land animal, will be more con- 

 veniently and intelligibly described in the same section. 



The greatest breadth of the pelvis of the Mylodon, taken across the iliac 

 plates, is three feet five inches : the antero-posterior diameter of the ilium is 

 one foot six inches. The labium of the ilium is continued in a curved line, de- 

 scribing about a third of a circle from the transverse processes of the last lumbar 

 vertebra to the anterior superior spinous process. From this point, the margin 

 of the bone is bent inwards with a sinuous outline to the neck of the acetabulum. 



The anterior or inner surface of the iliac plate is concave above and slightly 

 convex below ; it is traversed by a narrow longitudinal or vertical ridge ex- 

 tending towards the acetabulum, and doubtless indicating the boundary between 

 the psoas magnus and iliacus internus. 



A ridge* is continued from the posterior and upper margin of each ilium, 

 backwards and inwards upon the side of the sacrum, to the articular processes, 

 which are separate and distinct in the three last sacral vertebrae. These ridges 

 divide the channels between the spinal crest and articular processes, from the 

 wider and deeper channels situated between the articular processes of the pos- 



* PI. X. fig. 2. h. 

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