451 



The (lifters from the last two cases mentioned in two respects: 



1) In the relative position of the vessel that con- 

 stitutes the so-called ventral union (V. U.). This vessel 

 unites (Fig. 14) with the V. iliaca externa sinistra instead of with 

 the V. iliaca externa dextra as in Fig. 12, or instead of with the 

 Vv. iliacae externae at their point of union as in Fig. 10. 



2) In the absence of a union between the Vv. iliacae 

 inter nae ventral to the A. sacral is media. In Fig, 15 each 

 V. iliaca interna opens into the V. iliaca externa of its respective 

 side, dorsal to an A. iliaca communis, without a previous union 

 with its fellow of the opposite side ventral to the A. sacraHs media. 



Sul)-Tyx)e III, C. Those cases in which the above 

 mentioned dorsal and ventral unions are sub-e(iually 



d e V e 1 p e d. 



Two examples of this Sub-Type were met with distributed among 

 one male and one female. 



Fig, 16 is a ventral view of an example of this Sub-Tjq^e, The 

 Vv. iliacae internae, after uniting with each other ventral to the 

 A, sacralis media , unite with the 

 Vv, iliacae externae to form the 

 V. cava posterior by means of two 

 vessels of equal size, one of which 

 (the right) passes ventral, and 

 the other (the left) dorsal to an 

 A. iliaca communis. The dorsal 

 view of this example is omitted 

 since in appearance it is essen- 

 tially the same as the ventral view 

 (Fig. 16). 



Fig. 16. Ventral aspect. Adult ^j 

 Type III, A. For reference to abbreviations 

 see under Figs. 1 and 2. 



On the presence of a bifurcated V. cava posterior and 

 on the presence of foramina in the same in Didelphys virginiana. 



The following table shows the distribution of bifurcations and 

 also of perforations of the V. cava posterior among the twenty six 

 opossums examined (thirteen males and thirteen females). 



29* 



