64 



analogy of the remainder to those of the Megatherium, render it probable that 

 the sternum may have terminated in both animals by more simple bones, 

 completely detached from each other by the interposition of the extremities of 

 the posterior sternal ribs which were not bifurcated. 



Description of the Pelvis*. 



Sacrum. — The sacrum of the Mylodon, defined by its connections with the 

 ossa innominata, consists of seven vertebrae ; but according to the character of 

 anchylosis it includes eleven ; the last dorsal and the three lumbar vertebrje 

 being united with the ordinary sacrum by confluence of the neural arches and 

 spines, and, with the exception of the dorsal vertebra, also by continuous an- 

 chylosis of their bodies. 



The length of this enormous sacrum is two feet four inches ; it gradually 

 increases in breadth to the sacro-iliac symphysis, and, after a slight contraction, 

 again expands to join the ischia. The sychondrosal articulations of the sacrum 

 with both these parts of the ossa innominata are obliterated by continuous ossi- 

 fication. 



The body of the first lumbar or sacral vertebra is subcompressed, and its 

 sides converge to the inferior surface ; those of the second and third gradually 

 expand ; and in the third and following sacral vertebra the inferior surface is 

 broad, slightly concave, and meets the lateral surfaces at a right angle, from 

 which it is separated by a rough ridge on either side ; the breadth of the under 

 surface of the second true sacral vertebra, which is nearly flat, is three inches and 

 a half: this surface then slightly diminishes in breadth to the last sacral ver- 

 tebra. The longitudinal contour of the under surface of the sacrum describes a 

 shght double curvature, which is convex downwards, opposite the junction with 

 the iha, and concave at the other parts. The bodies of the lumbar and sacral 

 vertebrae are each perforated by two vascular canals extending from the middle 

 of the surface which supported the spinal marrow to the under surface. These 

 perforations assist in the completely confluent state of the bodies of the vertebrae 

 in determining their true number. 



* Plate X. 



