114 



the transverse process for the passage of tlie vertebral artery, but 

 mušt diligently compare them with those of others of the class, to 

 ascertain witli which they really correspond in thelr essential charac- 

 ters ; and then -vve may draw the line of demarcation wherever suits 

 us best, only remembering that under whichever series we place a 

 vertebra in one species, the corresponding one in another mušt be 

 reckoned under the šame category. This is the view I have endea- 

 voured to carry out in my examination of the Sloth ; and being of 

 opinion that the eighth and ninth vertebrse of that animal correspond 

 as essentially to the sixth and seventh in the ręst of the class, as do 

 the atlas and the axis to those of other animals, and knowing that 

 the intervening vertebrse diiFer in number by two, I feel bound to 

 believe, notwithstanding the interesting fact which Professor Bell 

 has discovered, that the cervical vertebrse of the Bradypus tridactyltis 

 are nine in number. 



Fig. 2. 



Fig. 1. A view froni behind of the seventh cervical vertebra of an Opossum {Didelphys Vir- 



gini(inu), as an example of the eiistence of the foramen for the passage of the vertebral 



artery, and showing the manner of its enclosure beneath. 

 Kg. 2. The sixth and seventh cervical, and the first tvvo dorsal vertebrae of a Polecat, showing 



the rudimentai rib attached to the lašt cervical. 

 Fig, 3. The series of seven cervical vertebree of a second specimen of the Polecat, shovving 



the absence of the rib, and the difference uffurm in the trumrerse processes. 

 Fig. 4. A perspective view (from behind) of the lašt cervical vertebra of the šame animal, 



8howine the absence of the forameu for the vertebral artery, and the fluttened form of 



the under surface of the vertebra. 



