The commencement of the dental canal in the Potoroos and Wombat is parallel with the beginning of the molar series, and it has the same relative position in the Stonesfield Mar supials; but in the other carnivorous and in sectivorous species the dental foramen is placed further back. In the Wombat a vascular groove is continued from the foramen along the inner side of the ramus of the jaw as in the Stones field fossils; and a corresponding but wider groove is present in the lower jaw of the Myr mecobius. In the Thylacine and Ursine Da syures and their fossil congener the Phascolo there, the condyle of the lower jaw is placed low down, on a level with the molar series: it is raised a little above that level in the smal ler Dasyures and Opossums, and ascends in proportion to the vegetable diet of the spe cies.
In all those Marsupials which have few or very small incisors the horizontal rami of the jaw converge towards a point at the symphysis. The angle of convergence is most open in the Wombat, and the gradual diminution in the size of the rami as they approach this part is most marked and direct. The internal surface of the symplaysis menti is almost horizontal, and is convex from side to side in the interval between the molars and incisors. The suture becomes obliterated in aged individuals. It is also wholly obliterated in the skull of a Koala now before me ; in all the other Marsu pial crania which I have examined, the mini of the lower jaw are disjoined at the sym physis; and in the Opossum, both the rami o the lower jaw and all the bones of the face a remarkable for the loose nature of their con nection.
Vertebral column.—The vertebral column i divisible in all the 'Marsupials into the usua classes of cervical, costal, lumbar, sacral, and caudal vertebrae. The cervical vertebra inva riably present the usual number, seven, an the usual character of the perforation of the transverse process, or rather the presence u the upper and lower transverse processes, and the union of their outer extremities with a ru dimental rib. I found the cervical ribs of th• dentata distinct and unanchylosed in a main Perameles. In the Dasyures, Opossums, Ban dicoots, and Phalangers, the seventh cervical vertebra has only the upper transverse process, and consequently wants the character of perforation, as in many of the ordinary Mani malia. In the Petaurists, Koala, Wombat Potoroos, and Kangaroos, the seventh vertebral is perforated like the rest. But in the Kan garoo both the dentata and atlas have the trans verse processes grooved merely by the vertebral 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 fested in the structure of the atlas (fig. 98), which exhibits a permanent paration of the neurapophyses or superior lamina from the centre or body below. In the ) Koala and Wombat the body of the atlas remains nently cartilaginous, and the lower part of the vertebral ring al by dried gristly substance In the Petaurists, Kangaroos, and Potoroos, the atlas is completed below by an extension of ossification from the neurapophyses 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, (Beute/thiere,, fig. c, plates iii. & vii.); but in some of the skeletons of these Marsu pials examined by me I find the ring com pleted and the fissure obliterated. In all the Marsupials 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 vertebra have their spines remarkably long and thick, but progressively diminishing from the third (fig. 101), 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 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 cervical vertebra 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 vertebra, the spines of which are much smaller and weaker than that of the dentata, and in the Plialangers 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 P ha langista Cookii that the superior flattened arches of the five last cervical vertebra 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 of the succeeding. This structure is ex hibited in a slighter degree in the cervical ver tebra 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 is 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 vertebra.