26 



of a tubercle, I long ago drew the inference that, like the Sloths and Auteaters, it was 

 not covered by a bony armour*. 



With regard to the cervical vertebrae, 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-f-. 



The atlas in both the Ai andUnau 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 vertebrae 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 vertebrae. In the Anteaters (Myrmecophaga) 

 the spine of the dentata is low and is extended more forwards than backwards ; the 

 spines of the other cervical vertebrae 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 vertebrae 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 which 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 



* Geological Transactions, 2nd Series, vol. vi. p. 101 (1839). 



t Description of the Fossil Mammalia collected during the Voyage of the Beagle, 4to. 1 838-40. pi. 20. p. 84. 



