_ arteries; and in the Koala and Wombat the 
atlas presents only the perforation on each side 
of the superior arch. In the Perameles and 
some other Marsupials, as the Cayopollin, an 
affinity to the cold-blooded Ovipara is mani- 
_ fested in the structure of the atlas (fig. 98), 
which exhibits a permanent se- 
paration of the neurapophyses 
or superior lamine from the 
centre or body below. In the 
Koala and Wombat the body 
of the atlas remains perma- 
nently cartilaginous, and the 
lower part of the vertebral ring 
‘is completed by dried gristly substance 
(fig. 99). In the Petaurists, Kangaroos, 
Fig. 99. 
Atlas of Pera- 
meles lagotis. 
Atlas, axis, and third cervical vertebra, Koala. 
_ and Potoroos, the atlas is completed below 
by an extension of ossification from the 
_ heurapophyses into the cartilaginous nucleus 
' representing the body, and the ring of the 
' vertebra is for a long time interrupted by a 
" longitudinal fissure in the middle line, the 
_ breadth of which diminishes with age. This 
_ fissure is represented in figures of the atlas of 
a Potoroo and Kangaroo, given by Pander and 
D’Alton, ( Beutelthiere, fig. c, plates iii. & vii.); 
but in some of the skeletons of these Marsu- 
pials examined by me I find the ring com- 
“taba and the fissure obliterated. In all the 
upials 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, (fig. 100), where it 
increases in thickness posteriorly; in these 
_ Species also the third, fourth, and fifth cervical 
_ vertebre have their spines remarkably long and 
_ thick, but progressively diminishing from the 
Ht 
pythird (fig. 101), which equals in height and 
be Fig. 100. 
be] 
“y 
a; Vertebra dentata, 
bs idelphys Virginiana. 
Third cervical vertebra, 
Didelphys Virginiana. 
MARSUPIALIA. 
277 
thickness, but not in longitudinal extent, the 
spine of the dentata. These spines are four- 
sided, and being closely impacted together, 
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 very long and strong, but have the 
ordinary sub-cylindrical rounded form. Tyson, 
who has described and figured the above struc- 
ture of the cervical vertebree in his anatomy of 
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 de- 
sign:” but this teleological conjecture is inva- 
lidated by the fact that the Phalangers, Petau- 
rists, Koala, and other arboreal Marsupials 
present the usual structure of the five posterior 
cervical vertebrz, the spines of which are much 
smaller and weaker than that of the dentata, 
and in the Phalangers and Petaurists almost 
obsolete. These do not require the neck to be 
strengthened to aid in overcoming the struggles 
of a resisting prey. I observe in the Pha- 
langista Cookii that the superior flattened 
arches of the five last cervical vertebre 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 vertebra, in the Opos- 
sum, is adapted to the strengthening and fix- 
ation 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 ex- 
ibited in a slighter degree in the cervical ver- 
tebre of the Dasyures, Phalangers, and Great 
Kangaroo. In the Petaurists, Potoroos, Wom- 
bat, 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 Poto- 
roos, Kangaroos, Petaurists, Phalangers, Opos- 
sums, and Dasyures, this process 1s remarkably 
expanded in the direction of the axis of the 
spine. In the Bandicoots corresponding pro- 
cesses are observed progressively increasing in 
size, on the fourth, fifth, and sixth cervical 
vertebre. 
The number of the dorsal vertebrz is greatest 
in the Wombat, where it is fifteen, correspond- 
ing with the number of pairs of ribs: it is 
least in the Petaurists, which have twelve 
dorsal vertebre.* In all the other genera there 
* In the skeletons both of the Pet. macrurus and 
Pet. sciureus in the Museum of the College of Sur- 
geons there are twelve pairs of ribs; but in the 
Pet. macrurus the succeeding vertebra has a short 
transverse process on each side, the extremity of 
which has the appearance of having supported a 
costal appendage. Cuvier, however, assigns but 
twelve dorsal vertebre to this species in his table. 
Leg. d’Anat, Comp, 2d edit. p. 180, 
