396 PROF. OWEN ON THE OSTEOLOGY 
spine being much longer ; those of the remaining dorsal vertebra 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 Perameles the centre of motion is at the eleventh dorsal vertebra ; in the 
Potoroo and Kangaroo at the twelfth ; in the Petaurists at the thirteenth vertebra. In 
the Phalangers, Opossums, Koala and Wombat the flexibility of the spine is much dimi- 
nished, and the centre of motion is not defined by the convergence of the spinous pro- 
cess towards a single vertebra, but they all incline slightly backwards. 
The lumbar vertebre are four in number in the Wombat, seven in the Petaurists, and 
six in other Marsupiata,—the total number of true vertebrz being thus the same in all 
the genera’. The pressure to which the trunk of the Wombat must occasionally be sub- 
jected, in its extensive subterranean burrowings, is probably the condition of the de- 
velopment of the additional pairs of ribs in that species. 
The anterior oblique processes, which begin to increase in length in the three poste- 
rior dorsal vertebre, attain a great size in the lumbar vertebre, 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 vertebre of all the other genera. The trans- 
verse processes of the lumbar vertebre progressively increase in length as the vertebra 
approach the sacrum ; they are most developed in the Wombat, where they are directed 
obliquely forwards. In the Kangaroos, Potoroos and Perameles they are curved for- 
wards and obliquely downwards. The length of these and of the anterior oblique pro- 
cesses is relatively least in the Petaurists, Phalangers and Opossums. 
The number of vertebra succeeding the lumbar, which are anchylosed together in 
the sacral region of the spine, amounts in the Wombat to seven ; but if we regard those 
vertebree only as sacral which join the ossa innominata, then there are but three. In 
the Phalangers there are generally two sacral vertebre, but in a small species from 
Tasmania (Phal. Cookit), the last lumbar assumes the character of the sacral vertebre, 
both by anchylosis and a partial junction with the ossa innominata. In the Kangaroos 
and Potoroos the impetus of the powerful hinder extremities is transferred to two an- 
chylosed vertebree. In the Perameles there is only a single sacral vertebra, the spine 
of which is shorter and thicker than those of the lumbar vertebre, and turned in the 
contrary direction, viz. backwards. In the Myrmecobius there are four sacral vertebrz 
by anchylosis, two of which join the ilia. In Mauge’s Dasyure two sacral vertebre are 
anchylosed ; but it is to the expanded transverse processes of the anterior one only that 
the innominata are joined. The same kind of union exists in the Viverrine Dasyure, but 
three vertebre are anchylosed together in this species. In the Phalangers and Pe- 
taurists there are two sacral vertebre. In Petawrus Taguanoides and Pet. macrou- 
' In Phalangista Cookii the sixth lumbar vertebra is joined by a part of its transverse process to the ossa in- 
nominata. 
