OF THE MARSUPIALIA. 395 
In all the Marsupiata the spine of the dentata is well developed, both in the 
vertical and longitudinal directions, but most so in the Virginian and Crab-eating 
Opossums, where it increases in thickness posteriorly ; in these species also the third, 
fourth and fifth cervical vertebra have their spines remarkably long and thick, but 
progressively diminishing from the third, which equals in height and thickness, but not 
in longitudinal 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 presents the same structure: in the Armadillos the corresponding spines 
are largely developed, but they are anchylosed together. In the Orang the cervical 
spines are remarkably developed in length, but have the ordinary, slender, subcylindri- 
cal, rounded form. Tyson, who describes and figures the above structure of the cervical 
vertebre 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. Unfortunately for this reasoning, however, the Phalan- 
gers, Petaurists, Koala, and other arboreal Marsupiata, whose arboreal habits render 
them equally liable to a fall, present the usual structure of the five posterior cervical 
vertebre, 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 
spinous processes are wanting in the five last cervical vertebrz of the Phascogale. 
The structure of the transverse processes of the cervical vertebrz, 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 obliquely, so that the posterior part 
of one transverse process overlaps the anterior part of the succeeding. This structure 
is exhibited in a slighter degree in the cervical vertebre of the Dasyures, Phalangers, 
and great Kangaroo. In the Petaurists, Potoroos, Wombat and Koala the direction 
and simpler form of the transverse processes 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 process is remarkably expanded in 
the direction of the axis of the spine. In the Perameles corresponding processes 
are observed, progressively increasing in size, in the fourth, fifth and sixth cervical 
vertebre. 
The number of the dorsal vertebre is greatest in the Wombat, where it is fifteen, 
corresponding with the number of pairs of ribs. In all the other genera they are 
thirteen. 
In the 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 
