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Megatherium ; the pelvis of which, in every differential feature, except the shorter 

 sacrum, manifests stronger and more massive proportions in the Megatherium 

 than in the Mylodon. 



The sacrum of the Scelidotherium resembles that of the Megatherium in the 

 number of component vertebra?, and in the free articulation of the first sacral 

 with the last lumbar vertebrse ; but in its expanded form, especially at the poste- 

 rior part where its transverse processes join the ischium, in its capacious medul- 

 lary cavity and wide nervous foramina, this part of the vertebral column in the 

 SceUdothere is conformable with that in the Mylodon and Megatherium, and 

 manifests a corresponding relation to the massive proportions and muscular 

 strength of the hind-legs. 



No authentic part of the sacrum or pelvis of the Megalonyx appears yet to 

 have been discovered. 



The resemblance which the pelvis of the Elephant bears to that of the Mega- 

 therium in the broad iliac bones, ceases there ; in the narrow sacrum and the 

 widely open ischiadic notches it adheres to the ordinary pachydermal type. In 

 fact the pelvis forms the most peculiar and characteristic part of the skeleton in 

 the Megatherian race ; it has been the centre from which muscular forces of very 

 unusual extent and vigour have emanated to act in different directions upon the 

 trunk and the anterior extremities, upon the hind limbs and the tail ; and the 

 resistance which these forces collectively, through the great pelvic fulcrum or 

 centre, have been employed to overcome, must have been of a very different 

 nature and degree to any that now opposes itself to the labours of existing vege- 

 table feeders in supplying their daily wants ; whence we may infer that the exer- 

 tion of such forces was associated witli equally peculiar habits in the Megatherioid 

 animals. 



Comparison of the Tail. 



The part of the skeleton in which the Sloths differ most obviously from the 

 Mylodon is the tail, which is so short as to be hardly visible in the entire animal. 

 But if the Unau shows a nearer affinity to the extinct Megatherioid animals in 

 certain features of its cranium and of the true and sacral vertebrae, it yields to 

 the Ai in regard to the caudal region, which is reduced to six short vertebrae in 

 the Bradypus didactylus, but numbers ten or eleven vertebrae, better developed 

 and of larger proportional size, in the Bradypus tridactylus. The transverse pro- 



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