60 



equal to that of the spinous process ; these pecuUar oblique processes have the 

 same relation to the spinous processes in the support of the osseous carapace, 

 which the tie-bearers have to the king-post in the architecture of a roof. 



As the only analogous trace of such developments from the neural arch is 

 presented by the low ridge or tubercle from the upper surface of the transverse 

 processes in the posterior dorsal vertebrae of the Mylodon and Megatherium, the 

 conclusion may be legitimately drawn from this single anatomical difference, 

 that the Megatherioids, like the Sloths and Ant-eaters, were not invested with a 

 bony carapace. 



The dorsal vertebrae of the Megatherium very closely resemble those of the 

 Mylodon ; but the anterior spines are relatively longer in order to give adequate 

 attachment to the supporters of the relatively larger and heavier head. The 

 dorsal vertebrae are equally remarkable with those of the Mylodon for the capa- 

 city of the spinal canal, the expanded arches of which similarly overlap each 

 other, and are articulated by broad, flat, and nearly horizontal surfaces : but 

 the following diiference is deserving of notice in the dorsal vertebrae of the 

 Megatherium, viz. that through a great proportion of the posterior part of the 

 dorsal region there is a third articular surface on both the front and back 

 parts of the imbricated neural arches ; the anterior surface is situated on the 

 anterior part of the base of the spine, between the two normal articular pro- 

 cesses ; the posterior median surface is supported on an osseous platform, 

 depending from the posterior and under part of the root of the spine : there is 

 a corresponding rough process in the Mylodon indicating a ligamentous union 

 of the imbricated arches, where the additional synovial joint existed in the 

 Megatherium. 



Notwithstanding the excess of either cervical or dorsal vertebrae which cha- 

 racterizes the existing Sloths, it is interesting to find that they agree with 

 both the Mylodon and Megatherium in the number of the lumbar vertebrae. 

 But the neural arches of these vertebrae are depressed, the transverse processes 

 short, the spines almost obsolete ; and all are unfettered by bony union in the 

 Sloths. The anchylosis of the lumbar vertebrae with each other and the sacrum 

 is as peculiar to the Mylodon amongst Mammalia, as is the number of cervical 

 vertebrae in the Ai. Strange that two such remarkable features in the osteology 

 of Birds as supernumerary cervical vertebrae with floating ribs, and confluence 



