738 OSSIFICATION OF VERTEBRA IN MARSUPIALS, 



will be found at once to break down, since in most marsupials the 

 upper caudal vertebrae have well developed autogenous transverse 

 processes which are undoubtedly serially homologous with the 

 pleurapophysial centres of those vertebra? which support the 

 pelvic bones. 



If the fifth vertebra of the sacro-caudal series of a half-grown 

 wombat be examined (fig. 9) it will be seen to be made up of a 

 well developed flattened centrum and a feebly developed arch, 

 with on each side a rather large, flattened autogenous transverse 

 process. The transverse processes articulate mainly with the 

 centrum, but also with the arches. A similar description would 

 apply to the 6th, 7th, and 8th vertebra?, but on reaching the 9th 

 the transverse process is found to articulate only with the 

 centrum. On passing forwards the 4th vertebra is found to be 

 very similar to the 5th, while the 3rd differs only in the slightly 

 increased development of the arch and of the transverse pro- 

 cesses. The second vertebra, which is usually regarded as a 

 true sacral vertebra, has the transverse processes strongly 

 developed for articulation with the ilia. In the first vertebra 

 of the sacro-caudal series the elements are exactly the same 

 as in the fifth vertebra and only differ in being larger, and 

 in having the arch proportionately more largely developed, and 

 in having the lateral elements specialised for the support of the 

 pelvic bones. In fig. 5 is shown a front view of the first sacro- 

 caudal vertebra, and well displaying the distinctness of the 

 lateral elements. In fig. 6 is seen a similar view of the first 

 sacro-caudal vertebra of Deuterosaurus, where the lateral element 

 remains distinct apparently throughout life. 



In Dideljjhys the condition is very similar to that in Phasco- 

 lomys, as may be seen in fig. 10 ; the transverse processes, how- 

 ever, of the caudal vertebra? are scarcely so well developed. 



In Macropus the first two vertebra? have well-developed lateral 

 elements ; the third has the lateral element small but distinct ; 

 the fourth has a large autogenous process ; while the fifth has a 

 small autogenous process. The other vertebra? of the series do 

 not appear to have autogenous processes. 



