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longitudinal directions, but most so in the Virginian and Crab-eating 

 Opossums, where it increases in thickness posteriorly; in these spe- 

 cies also the third, fourth, and fifth cervical vertebrae have their 

 spines remarkably long and thick, but progressively diminishing 

 from the third, which equals in height and thickness, but not in lon- 

 gitudinal extent, the spine of the dentata. These spines are four- 

 sided, and being closely impacted one behind another must add 

 greatly to the strength while they diminish the mobility of this part 

 of the spine. I know of no other Mammiferous genus which pre- 

 sents the same sti-uctui-e : in the Armadillos the coi-responding spines 

 are largely developed, but they are anchylosed together. In the 

 Orang the cervical spines are remarkably developed, but have the 

 ordinary slender subcylindrical rounded form. Tyson, who describes 

 and particularly figures the above structure of the cervical vertebrae 

 in the Opossum, conjectures that it is given to this arboreal animal 

 in order that there might be ' no danger of its breaking its neck 

 should it happen to fall to the ground by chance or design.' Un- 

 fortunately for this reasoning, however, the Phalangers, Koala, and 

 other Marsupiata, whose arboreal habits render them equally liable 

 to a fall, present the usual structure of the five posterior cervical 

 vertebrae, the spines of which are all much less than that of the 

 dentata, and in the Phalangers and Petaurists almost obsolete. I 

 observe in the Phalangista Cookii that the superior flattened arches 

 of the five last cervical vertebrae bear a ridge on each side of the 

 spine, having the same direction and form, and nearly the same size. 

 The structure of the transverse processes of the cervical vertebrae, in 

 the Opossum, is adapted to the strengthening and fixation of this part 

 of the vertebral column ; they are exj.'anded nearly in the axis of the 

 spine, but obliquely, so that the posterior part of one transverse pro- 

 cess overlaps the anterior part of the succeeding. This structure is 

 exhibited in a slighter degree in the cervical vertebrae of the Dasyures, 

 Phalangers, and Great Kangaroo. In the Petaurists, Potoroos, Wom- 

 bat, and Koala, the direction and simpler form of the transverse pro- 

 cesses allows of greater freedom of lateral motion. In the Koala and 

 Wombat a short obtuse process is given off from the under part of the 

 transverse process of the sixth cervical vertebra. In the Potoroos, 

 Kangaroos, Petaurists, Phalangers, Opossums, and Dasyures, this pro- 

 cess is remarkably expanded in the direction of the axis of the spine ; 

 in the Perameles corresponding processes are observed progressively 

 increasing in size, on the fourth, fifth, and sixth cervical vertebrae. 

 " The number of the dorsal vertebrae is greatest in the Wombat, 

 where it is fifteen, corresponding with the nuraljer of pairs of ribs ; 

 it is least in the Petaurists which have twelve dorsal vertebrae. 

 In all the other genera there are thirteen. In tlie Koala the 

 length of the spine of the first dorsal hardly exceeds that of the last 

 cervical ; but in all other Marsupials the difference is considerable, 

 the first dorsal spine being much longer ; those of the remaining 

 dorsal vertebrae progressively diminish in length, and increase in 

 breadth and thickness. They slope backwards towards the centre 

 of motion. In Mange's Dasyure this is sliown to be at the uintli 



