BY R. BROOM. 739 



In Trichosurus the first two vertebrae have large autogenous 

 lateral elements, and a considerable number of the succeeding 

 vertebrae have small autogenous transverse processes. 



As the lateral pleurapophysial elements of the first sacral 

 vertebra are thus seen to be homologous with the autogenous 

 lateral elements of the succeeding vertebrae, it becomes quite 

 impossible to draw a distinction between sacral and caudal verte- 

 brae by the criterion above referred to. 



In the manati and the beaver among Eutherians the trans- 

 verse processes of the caudal vertebrae are developed auto- 

 genously ; but Flower doubts if " this circumstance alone is suffi- 

 cient to entitle them to be considered as costal elements." From 

 the marsupial condition it is manifest that the caudal transverse 

 elements are homologous with the lateral elements of the first 

 sacral vertebra, and as it is pretty well established by comparative 

 anatomy that the lateral elements of the sacrum are modified 

 ribs, we are forced to the conclusion that the autogenous trans- 

 verse processes of the upper caudal vertebrae in the marsupials 

 are really costal elements. 



EXPLANATION OF PLATE XLIX. 



Fig. 1. — Front view of 3rd lumbar vertebra of half-grown wombat. 



Fig. 2. — Front view of 4th lumbar vertebra of half -grown wombat. 



Fig. 3. — Upper view of 4th lumbar vertebra of half -grown wombat. 



Fig. 4. — Upper view of a lower trunk or lumbar vertebra of Cynognathus 

 (reduced ; modified from Seeley). 



Fig. 5. — Front view of 1st sacro-caudal vertebra of half -grown wombat. 



Fig. 6. — Front view of 1st sacro-caudal vertebra of Deuterosaurus (reduced ; 

 modified from Seeley). 



Fig. 7.— Under view of 1st sacro-caudal vertebra of half -grown wombat. 



Fig. 8. — Under view of 5th and 6th sacro-caudal vertebra; of half -grown 

 wombat. 



Fig. 9. — Under view of 2nd, 3rd, and 4th sacro-caudal vertebrae of half- 

 grown wombat. 



Fig. 10. — Under view of first four sacro-caudal vertebrae of young Didelphys 

 (x 4). 



