443 



So many variations of this type were met with that a further 

 sub-division of Type III was found necessary, as follows : 



Sub-Type III, A. Those cases in which the principal 

 union between the Vv. iliacae intern ae and ex- 

 te rnae lies ventral to the arteries in question. 

 Sub-Type III, B. Those cases in which the principal 

 union between the Vv. iliacae inter nae and ex- 

 ternae lies dorsal to the arteries in question. 

 Sub-Type III, C. Those cases in which the above 

 mentioned dorsal and ventral unions are sub-equally 

 developed. 

 The following table shows the distribution of the above men- 

 tioned Types among the twenty six individual opossums examined 

 (thirteen males and thirteen females): 



Although possibly a sufficient number of opossums have not 

 been examined to determine the question, at the present writing there 

 appears to be no relation between sex or age and any particular 

 mode of formation of the V. cava posterior. 



Before describing the different types mentioned above, it may 

 be said that almost without exception, in every individual examined, 

 there was some marked variation within the Type, either in the 

 arrangement of the vessels to form the V. cava posterior or in con- 

 nection with the tributaries of the Vv. iliacae. 



A detailed description of these variations within the Type will, 

 for the most part, be reserved for a future publication. 



Description of the formation of tlie V. cava posterior 

 in Didelphys virginiana. 

 Type I. Those cases in which the Vv. iliacae in- 

 te rnae unite with the Vv. iliacae externae to 

 form the V. cava posterior, ventral to the A a. 

 iliacae communes or ventral to the Aorta. 



