PROFESSOR OWEN ON THE MEGATHERIUM. 
381 
summit, fig. 12. This simplified modification of the central element terminates the 
vertebral series. 
Comparison of the V ertehral Column. 
In the number of the true vertebrae, as well as of their kinds, the Myrmecophaga 
juhata, amongst the Edentata, 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, Choloepus 
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 hrevicaudata) has seven cervical and sixteen dorsal 
vertebrae, but it differs from the Megatherium in having five lumbar vertebrae. The 
Armadillo tribe (Dasypodidce) 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. 
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 
Myrmecophagce : 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 XVII. and Plate XIX. figs. 4 and 5, m, not being developed beyond the 
state of a tubercle, I long ago drew the inference that, like the Sloths and Anteaters, 
it was not covered by a bony armour-l-. 
* See Plate XLIX. figs. 18 & 19, m, of Part I. of this memoir. Philosophical Transactions, 1851. 
t Geological Transactions, 2nd Series, vol. vi. p. 101 (1839). 
