Zoological Society. 



433 



by the presence or absence of articulated ribs, nor of a foramen in 

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

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

 ascertain with which they really correspond in their essential charac- 

 ters ; and then we 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 must be 

 reckoned under the same 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 vertebrae of that animal correspond 

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

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

 the intervening vertebrae differ in number by two, I feel bound to 

 believe, notwithstanding the interesting fact which Professor Bell 

 has discovered, that the cervical vertebrae of the Bradypus tridactylus 

 are nine in number. 



Fig. 2. 



Fig. 3. 



Fig. 1. A view from behind of the seventh cervical vertebra of an Opossum {Dide.lphys Vir- 



giniana), as an example of the existence of the foramen for the passage of the vertebral 



artery, and showing the manner of its enclosure beneath. 

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



the rudimental rib attached to the last cervical. 

 Fig. 3. The series of seven cervical vertebrae of a second specimen of the Polecat, showing 



the absence of the rib, and the difference of form in the transverse processes. 

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



showing the absence of the foramen for the vertebral artery, and the flattened form of 



the under surface of the vertebra. 



Ann. fy Mag. N. Hist. Vol. xx. SuppL 



31 



