25 



sented by an obtuse ridge on each side of the centrum. In the last centrum, figs. 12, 

 13, all the processes have disappeared, and it presents the form of a low rounded 

 cone, with a smooth concave pentangular base, fig. 13, and a rough tuberous 

 summit, fig. 12. This simplified modification of the central element terminates the 

 vertebral series. 



fy 3. Comparison of the Spinal Column with that of other Bruta. 



In the number of the true vertebra?, and of those in each kind, the Myrmecophaga 

 jubata, amongst the Bruta, agrees with the Megatherium. The Ai, or Three-toed 

 Sloth, Bradypus tridactylus, has the same number of dorsal and lumbar vertebrae, 

 but has two more in the cervical region; the Unau, or Two-toed Sloth, Cholaipus 

 didactylus, has the same number of cervical and lumbar, but has eight additional 

 dorsal vertebrae, being the greatest number known in any mammalian quadruped. 

 The Short-tailed Manis (Manis brevicaudata) has seven cervical and sixteen dorsal 

 vertebrae, but it differs from the Megatherium in having five lumbar vertebrae. The 

 Armadillo tribe (Dasypodida?) differ most from the Megatherium in the inferior num- 

 ber of the dorsal vertebrae, which do not exceed eleven in some species, nor twelve in 

 any. The Orycterope, Orycteropus capensis, shows its affinity to the Armadillos in 

 having but thirteen dorsal vertebrae : and, like them, it has five lumbar vertebrae. 

 With regard to the structure of the vertebrae, the Anteaters, both hairy (Myrmeco- 

 phaga) and scaled (Manis) most resemble the Megatherium in the length and the 

 uniform backward inclination of the spinous processes ; but these processes are not so 

 long in proportion to their antero-posterior extent. The spinous processes of the dorsal 

 vertebrae are short and, in the hinder ones, obsolete in the Sloths : the Unau shows 

 the nearest affinity to the Megatherium by having a few of the anterior dorsal spines 

 better developed than in the Ai. In the Orycterope the last dorsal spine is vertical, 

 indicating a centre of motion in the trunk, those behind and those before slightly 

 converging towards this centre : nothing like this appears in the Megatherium. 



In the development of the accessory articular surfaces upon both anapophysis and 

 parapophysis of the last dorsal and lumbar vertebrae, the Megatherium manifests a 

 more direct departure from the Sloths and a proportionate affinity to the Anteaters. 

 The Armadillos, which likewise possess these accessory joints, have superadded pecu- 

 liarities of the posterior dorsal and lumbar vertebrae, in relation to the support of 

 their peculiar bony armour, of which the Megatherium offers as little trace as do the 

 Myrmeeophagae : I allude to the progressively and rapidly increasing length of the 

 metapophysis *. These, in the lumbar region, equal in length the spinous process 

 itself; to which the metapophyses bear the same relation in the support of the over- 

 arched carapace that the tie-bearers do to the king-post in the architecture of a roof. 

 From the fact of the metapophyses in the dorsal and lumbar vertebrae of the Mega- 

 therium, Plate I. and Plate III. figs. 4 and b,m, not being developed beyond the state 



* See Plate XLIX. figs. 18 & 19, m, Philosophical Transactions, 1851. 



D 



