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

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

 cies also the third, fourth, and fifth cervical vertebra- have their 

 spines remarkably long and thick, but progressively diminishing 

 froin the third, ■vvhich 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 niust add 

 greatly to the strength while they dimiuish the raobillty of this part 

 of the spine. 1 know of no other Mammiferous genus vvhich pre- 

 sents the šame structure : in the Armadillos the corresponding 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, conjectui'es that it is given to this arboreal animal 

 in order that there raight 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 eąually 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 alraost obsolete. I 

 observe in the Phalangista Cookii that the superior flattened arches 

 of the five lašt cervical vertebras bear a ridge on each side of the 

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

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

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

 of the vertebral column ; they are expanded nearly in the axis of the 

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

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

 exhibited in a slighter degree inthe cervical vertebrse 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 oiFfrom the under part of the 

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

 Kangaroos, Petaurists, Phalangei's, 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 vertebrse is greatest in the "VVombat, 

 where it is fifteen, corresponding with the number of pairs of ril)s ; 

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

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

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

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

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

 dorsal vertebrae progressively diminish in length, and increasc in 

 breadth and thickness. They slope backvvards tovvards the centre 

 of motion. In Mauge's Dasyure this is shovvn to be at the ninth 



