396 Venous System of Didelphys Marsupialis 
(V.1.e.). In Text Fig. VII the internal iliac veins (V.11.),, after 
anastomosing with each other ventral to the A. sacralis media (A.s.m.), 
join the external iliac of the left side (V.i.e.) by means of a common 
internal iliac vein. In Text Fig. VIII the posteava is partially bifur- 
cated and each internal iliac vein (V.1.1.) unites with the external iliac 
(V.i.e.) of its own side to form a common iliac vein (V.i.¢.) without 
previously anastomosing with the vein of the opposite side, ventral to 
the A. sacralis media. 
Fie. VII. HirGemyelelile 
Fics. VII and VIII. Two abnormalities of the postrenal division of the post- 
cava of Didelphys marsupialis. Ventral views. 
A.i.c., A. iliaca communis; A.i.e., A. iliaca externa; A.i.i., A. iliaca interna; Ao., 
Aorta; A.s.i.a., A. spermatica interna anterior; A.s.i.p., A. spermatica interna poster- 
ior; A.s.m., A. sacralis media; pc., posteava; V.i.c., V. iliaca communis; V.i.e., V. 
iliaca externa; V.i.i., V. iliaca interna; V.il., V.iliolumbalis, V.r., V.renalis; V.s.m., 
V. sacralis media; V. sp.i., V. spermatica interna. 
In view of the circumstance that variations in the mode of origin of 
the posteava are not exceptional but are the rule in Didelphys, and that 
these two particular cases differ fundamentally from the usual Types of 
variation, they must be regarded as the only genuine abnormalities that 
were met with among the one hundred and one opossums examined. 
An explanation of the development of these two abnormalities will 
be deferred until the second part of this paper. It appears to the writer 
as worthy of mention, however, that the morphological explanation of 
these two abnormalities may be the same as that which accounts for the 
