334 ABSTRACTS OF PAPERS. [VOL. II. 



In all marsupials hitherto examined (Petaurus taguanoides 

 excepted) the posterior vena cava has been found to lie ventral 

 to the aorta between the renal and common iliac veins, and to 

 be formed through a union of the common iliac veins, which 

 takes place ventral to the arteries. 



In DidelpJiys virginiana the posterior vena cava is not formed 

 in this manner. 



In fact, the mode of formation of the posterior vena cava 

 was found to be so variable in DidelpJiys virginiana that it is 

 quite impossible to assign any one mode of origin for this 

 vessel which may be regarded as typical of the species. 



For descriptive purposes the various modes of origin of the 

 posterior vena cava in DidelpJiys have been classified by the 

 writer under three main types as follows : 



Type I includes those cases in which the internal iliac veins 

 unite with the external iliac veins to form the posterior vena 

 cava, ventral to the common iliac arteries or ventral to the 

 aorta. 



Type II includes those cases in which the internal iliac 

 veins unite with the external iliac veins to form the posterior 

 vena cava, dorsal to the common iliac arteries or dorsal to the 

 aorta. 



Type III includes those cases in which the internal iliac 

 veins unite with the external iliac veins to form the posterior 

 vena cava, both dorsal and ventral to the common iliac arteries 

 or both dorsal and ventral to the aorta. 



So many variations of this type were met with that a 

 further subdivision of Type III was found necessary, as 

 follows : 



Type III, A, includes those cases in which the principal 

 union between the internal and external iliac veins lies ventral 

 to the arteries in question. 



Type III, B, includes those cases in which the principal 

 union between the internal and external iliac veins lies dorsal 

 to the arteries in question. 



Type III, C, includes those cases in which the above- 

 mentioned dorsal and ventral unions are sub-equally de- 

 veloped. 



