382 
PROFESSOR OWEN ON THE MEGATHERIUM. 
With regard to the cervical vertebrse, the fact of the Megatherium having the 
normal number in the Mammalian class, seven — if it were not sufficiently established 
by the well-adjusted articulations of those in the skeleton here described, rendering 
any supplemental vertebrae inadmissible, — would have been made most probable by 
the same number being present in the skeleton of the Megatherium at Madrid, and 
in the more complete skeleton of the Mylodon in the Museum of the Royal College 
of Surgeons in London. Moreover, that one of the Megatherioids had seven cervical 
vertebrae and no more is certain : the skeleton of the Scelidotherium, discovered and 
deposited by Mr. Darwin in the Museum of the Royal College of Surgeons, having 
been imbedded, without disturbance of the true vertebrae, and those of the neck being 
exposed in the ordinary number, and in their natural juxtaposition, on the removal 
of the stony matrix*. 
The atlas in both the Ai and Unau presents but two perforations on each side upon 
the upper surface ; one in front, the other behind the base of the transverse process, 
and this is less produced and is of a quadrate rather than a triangular form. 
The dentata of the Unau resembles more that of the Megatherium in the size of 
the spinous process than that of the Ai does ; but the spine in the Unau is pointed 
behind, not bifurcate. In the forms and proportions of the spines of the succeeding 
cervical vertebrse the Unau approaches nearer to the Megatherium than does any 
other existing Edentate species ; but the spine of the seventh cervical is by no means 
proportionally so developed, and metapophyses are not present in any. The Arma- 
dillos are distinguished from all other Bruta by the great breadth, the shortness and 
the anchylosis of the middle cervical vertebrse. In the Anteaters {Myrmecophaga) 
the spine of the dentata is low and is extended more forwards than backwards ; the 
spines of the other cervical vertebrse are still less elevated. In the long-tailed 
Manis very similar proportions of the cervical spines prevail. 
The closest correspondence with the Megatherium in the form and structure of the 
cervical vertebrse is presented as might be expected by its extinct congeners, the 
Mylodon and Scelidotherium. 
A resemblance of the Armadillos to the Megatherium has been pointed out in the 
ossification of the sternal ribs, but this is a character common to the order Edentata, 
and is consequently equally manifested by the Sloths. The Anteaters most resemble 
the Megatherium in the double joints by which the sternal ribs articulate with the 
sternum. There is, however, a character by whieh the Sloths peculiarly resemble the 
Megatherium, viz. in the anchylosis of the sternal with the vertebral portion of the 
rib in those of the first three dorsal segments. The Unau most resembles the Mega- 
therium in the form of the manubrium sterni, having the same prolongation of that 
bone in advance of the expanded part giving articulation to the first rib. Tliis pro- 
longation, which is not present in the Bradypus tridactyhis, relates to the complete 
* Description of the Fossil Mammalia collected during the Voyage of the Beagle, 4to. 1838-40. pi. 20. 
