57 



having the transverse process compressed obhquely, whilst in the Ai it is more 

 horizontal, as in the Mylodon. I find in the Ai only two vascular perforations 

 on each side the atlas, both of which are above the root of the transverse pro- 

 cess. 



The spinous process of the dentata corresponds in the Unau, both in size and 

 shape, with that of the Mylodon ; it is relatively smaller and inchned forwards in 

 the Ai. The slender spine of the third vertebra is overlapped by the large spine 

 of the second in the Unau, as in the Mylodon ; while it is equidistant from the 

 adjoining spines, and equal with those which succeed it in the Ai. 



Other Edentata besides the Unau resemble the Mylodon in the number of 

 cervical vertebrae, but not more so than other Mammalia ; a brief glance at the 

 peculiarities of these vertebrte in Armadillos and Ant-eaters will suffice to show 

 that they differ in this part of their skeleton more than the Unau from the My- 

 lodon. The Chlamyphore and other Armadillos are at once distinguished by the 

 anchylosis of a certain number of the cervical vertebraj. In the Ant-eaters the 

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

 those of the other cervicals are still less elevated. In the long-tailed Manis very 

 similar proportions of the cervical spines prevail ; but in the short-tailed species, 

 and in the Orycteropus the cervical spines approach nearer to the forms and pro- 

 portions of those in Mylodon, yet not so close, on account of their greater antero- 

 posterior extent, as in the Unau. 



It is, however, in the extinct MamraaUa, and precisely in those whose den- 

 tition and cranial organization prove them to belong to the same natural family 

 as the Mylodon, that the closest correspondence prevails in the form and struc- 

 ture of the cervical vertebrae. 



In the large proportion of the skeleton of the Scelidothere, discovered by 

 Mr. Darwin in the limestone of Bahia Blanca, the whole cervical and the ante- 

 rior part of the dorsal series of vertebrae were imbedded in natural juxtaposition ; 

 and the seven cervical vertebrae closely correspond in the form and propor- 

 tions of their spinous processes with those of the Mylodon*. The transverse 

 processes of the atlas are as remarkable as in the Mylodon for their great length, 

 breadth and thickness, and equally indicate the muscular forces passing from 



* Zoolog)' of the Beagle, ' Fossil Mammalia," p. 84. pi. 24. 

 H 



