278 



MARSUPIALIA. 



are thirteen dorsal vertebras 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 Mauge'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- 

 meles 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 

 vertebrae being thus the same in all the genera.-f 

 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- 

 tebra?, 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 vertebrae of all the other genera. 

 The transverse processes of the lumbar vertebrae 

 progressively increase in length as the verte- 

 brae 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 vertebrae 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 vertebras only as sacral which join the ossa 

 innominata, then there are but three. In the 

 Phalangers there are generally two sacral ver- 

 tebra?, but in the Phalangista Cookii the last 

 lumbar assumes the character of the sacral ver- 



* Cuvier assigns only twelve dorsal vertebrae to 

 the Kangaroo Rat, but in two different species of 

 Hypsiprymnus in the Mus. Coll. Surgeons, there 

 are thirteen dorsal and six lumbar vertebrae, and 

 I observed the same number in the skeletons of 

 Hypsiprymmis ursinus and dorcocephalus in the 

 Leyden Museum. Pander and D' Alton figure 

 thirteen dorsal vertebras in the Hypsiprymnus mu- 

 rinits. 



f In Phal. Cuuhii the sixth lumbar vertebra is 

 joined by a part of its transverse processes to the 

 ossa iunoj^jnata. 



tebrae both by anchylosis and partial junction 

 with the ossa innominata. 



Fig. 102. 



Pelvis of the Wombat. 



In the Kangaroos and Potoroos the impetus 

 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 vertebrae, and is turned in the contrary 

 direction, viz. backwards. 



In the Myrmecobius there are four sacral 

 vertebrae by anchylosis, two of which join the 

 ilia. In Mauge's Dasyure, two sacral ver- 

 tebrae 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 vertebrae are anchylosed to- 

 gether in this species. In the Phalangers 

 and Petaurists there are two sacral vertebrae. 

 In Petuurus macrurus three are anchylosed 

 together, though only two join the ilium. In 

 the Wombat (Jig. 102) the transverse pro- 

 cesses of the numerous anchylosed vertebrae 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 

 vertebrae. The transition from the sacral to the 

 caudal vertebrae is very obscure in the Wombat. 

 If we limit the sacral to the three which join 

 the ilium, then there remain twelve vertebrae 

 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 vertebrae, 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 Chaeropus 

 it seems to be wanting. In one species of 

 Perameles I find eighteen caudal vertebrae ; in 



