The number of the dorsal vertebra 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 vertebra.*' In all the other genera there are thirteen dorsal vertebra and thirteen pairs of ribs.* 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 diffe rence 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, which in Mange's Dasyure is shown to be at the ninth dorsal vertebra, by the verticality of its spine, towards which both the preceding and succeeding spines incline. In the Pera tneles the centre of motion is at the eleventh dorsal vertebra, in the Potoroo and Kangaroo at the twelfth, in the Petaurists at the thir teenth vertebra. In the Phalangers, Opossum, Koala, and Wombat the flexibility of the spine is much diminished, and the centre of motion is not defined by the convergence of the spinous process towards a single vertebra, but they all incline slightly backwards.
The lumbar vertebrae are four in number in the Wombat, seven in the Petaurists, and six in other Marsupialia ; the total number of true vertebrmbeing thus the same in all the genera.t The pressure which the trunk of the Wombat must occasionally have to resist in its extensive subterranean burrows, is probably the condition of the development of the additional pairs of ribs in that species.
The anterior oblique processes, which begin to increase in length in the three posterior dorsal vertebrae, attain a great size in the lumbar ver tebrae, and are locked into the interspace of the posterior oblique processes which are double on each side, except in the Perameles, and in the last lumbar vertebra of all the other genera. The transverse processes of the lumbar vertebra progressively increase in length as the verte bra approach the sacrum ; they are most deve loped in the Wombat, where they are directed obliquely forwards. In the Kangaroos, Poto roos, and Perameles, they are curved forwards and obliquely downwards. The length of these and of the anterior oblique processes is rela tively least in the Petaurists, Phalangers, and Opossums.
Sacrum.—The number of vertebra succeeding the lumbar which are anchylosed together in the sacral region of the spine, amounts in the Wombat to seven (jig. 102), but if we regard those vertebra only as sacral which join the ossa innominata, then there are but three. In the Phalangers there are generally two sacral ver tebrae, but in the Phalangista Cookii the last lumbar assumes the character of the sacral ver tebra both by anchylosis and partial junctil with the ossa innominata.
In the Kangaroos and Potoroos the itnpetns4 of the powerful hinder extremities is transferred to two anchylosed vertebrae. In the Perameles there is only a single sacral vertebra, the spine of which is shorter and thicker than those of the lumbar vertebra, and is turned in the contrary direction, viz. backwards.
In the Myrmecobius there are four sacral vertebra: by anchylosis, two of which join the ilia. In Mange's Dasyure, two sacral ver
tebra are anchylosed, but it is to the expanded transverse processes of the anterior one only that the ossa innominata are joined. The same kind of union exists in the Viverrine Da syure, but three vertebra are anchylosed to gether in this species. In the Phalangers and Petaurists there are two sacral vertebra. In Petaurus tnacrurus three are anchylosed together, though only two join the ilium. In the Wombat (fig. 102) the transverse pro cesses of the numerous anchylosed vertebra are remarkable for their length and flatness, those of the first four are directed outwards and are confluent at their extremities; the remaining ones are turned in a slight degree backwards, and very nearly reach the tuberosities of the ischia, behind which they gradually diminish in size and disappear in the three last caudal vertebra. The transition from the sacral to the caudal vertebra is very obscure in the Wombat. If we limit the sacral to the three which join the ilium, then there remain twelve vertebral for the tail. The spinal canal is complete in all but the last three, which consist only of the body. There are no inferior spines, and as only the six posterior vertebra, which progressively diminish in length, extend beyond the posterior aperture of the pelvis, the tail is scarcely visible in the living animal. In the Koala (fig. 109) the tail is also very short. In the Chwropus it seems to be wanting. In one species of Perameles I find eighteen caudal vertebra; in another twenty-three. In two species of Po toroo there are twenty-four caudal vertebra, but the relative length of the tail differs in these by one-third, in consequence of the different length of the bodies of the vertebra. In Hypsiprymnus ursinus there are more than twenty-six caudal vertebrae. In the great Kan garoo there are twenty-two caudal vertebra. In Bennett's Kangaroo there are twenty-four caudal vertebra, which are remarkable for their size and strength. In the Phwlangista vulpine, there are twenty-one caudal vertebra. In the Petaurus maerurus I find twenty-eight caudal vertebrae, while in the Pet.sciureus there are but twenty ; the bodies of the middle‘cau dal vertebra in both these species are remarkably long and slender.- The Myrmecobius has twenty-three caudal vertebra. In the Dasyurus Maugei I find twenty caudal vertebrae; in Dideiphis can crivora there are thirty-one; in the Virginian Opossum there are twenty-two caudal vertebra. In the latter species the spinal canal is con tinued along the first six ; beyond these the superior spinou s processes cease to be developed, and the body gives off, above, only the two anterior and two posterior oblique processes, which are rudimental, and no longer subservient to the mutual articulation of the vertebra. The transverse processes are single on the first five caudal vertebra, and are nearly the breadth of the body, but diminish in length from the second caudal, in which vertebra they are gene rally the longest. In the other vertebra a short obtuse process is developed at both extremities of the body on either side, so that the dilated articular surfaces of the posterior caudal ver tebra present a quadrate figure.