736 OSSIFICATION OF VERTEBRAE IN MARSUPIALS, 



Trichosurus the two arches meet inferiorly without a third 

 element, but occasionally a rudimentary inferior element is 

 present. In the smaller wallabies the arches meet as in 

 Trichosurus. In the larger wallabies and kangaroos, in the 

 wombat, in Phascolarctus, and in the large extinct Diprotodonts, 

 there is a more or less wide gap between the lower ends of the 

 arches bridged by fibrous tissue. It would seem as if in the 

 smaller and ancestral Diprotodonts the intermediate piece had 

 become gradually reduced in size until it became lost, and that 

 as the Diprotodonts increased in size the arches became again 

 separated, the place of the lost intermediate piece being taken by 

 fibrous tissue. 



The axis vertebra is very similar in structure to that in the 

 higher forms. There is, however, one interesting point of 

 difference in the development, in that whereas in man, and pro- 

 bably most of the higher mammals, the odontoid process is ossified 

 from a pair of centres, in the marsupials there is but a single 

 median centre as in the centra of the more normal vertebrae. 



The cervical vertebrae from the 3rd to the 7th are ossified from 

 three centres — one for the body and one for each arch. I have 

 been unable to find any ossification which could be regarded as a 

 costal element. 



Dorso-lumbar Vertebrce. — The dorsal vertebrae are developed 

 similarly to those in the higher mammals; and in the majority of 

 marsupials the same may be said of the lumbar vertebrae. In the 

 wombat (Phascolomys mitchelli), however, a remarkably interesting 

 exceptional condition is met with. The first three lumbar 

 vertebrae are developed from three centres as in man. but the 

 fourth lumbar vertebra differs in having well marked autogenous 

 transverse processes. In figs. 2-3 of Plate xlix., are shown 

 anterior and upper views of the fourth lumbar vertebra of a half 

 grown wombat. When compared with the third lumbar vertebra 

 (fig. 1), the only difference of any importance is that seen in the 

 condition of the transverse processes. In the third vertebra there 

 is a fairly large transverse process developed exogenously from the 

 arch, while in the fourth vertebra there is a small transverse 



