Charles F. W. McClure 399 
in any of the opossums examined. When the postcava was bifureated 
as far craniad as the internal spermatic veins one or both of the posterior 
internal spermatic arteries passed ventrad between the two veins (Figs. 
epi t,o. Lv and ed deletes die I andy )). 
In five opossums (3 males and 2 females) the postcava was only par- 
tially bifureated caudad of the imternal spermatic veins, as in Figs. 18 and 
25, Plates IV and V). 
In nine opossums (1 male and 8 females). as in Figs. 10 and 15, Plates 
II and IV), a foramen was present in the postcava at the level of the 
internal spermatic veins through which the posterior internal spermatic 
arteries passed, and the fusion between the caudal ends of the postcaval 
veins was incomplete. In one case a posterior mesenteric artery passed 
through the foramen instead of the spermatic arteries (Text Fig. IX). 
In seven opossums (2 males and 5 females) there were no indications 
of a previous bifurcation except for the presence of a foramen in the 
postcava, which was situated at the level of the internal spermatic veins, 
and through which one or both of the posterior internal spermatic arteries 
passed (Figs. 16 and 20, Plate IV). 
In the specimen of Petrogale examined by the writer the left posterior 
internal spermatic artery (A. s. 1. p.) also passed through a foramen in 
the postcava which, as in Didelphys, was situated at the level of the 
internal spermatic veins (V.sp.i.) (Text Fig. VI). 
THe RELATIONS WHICH Exist BETWEEN THE DIFFERENT TYPES OF 
VARIATIONS AND THE PRESENCE OF A BIFURCATED POSTCAVA. 
As shown by the table, the postcava in the adult was most frequently 
bifurcated or presented indications of a previous bifurcation, in those 
animals in which the internal iliac veins make their principal union with 
the external iliacs to form the postcava by means of vessels which le 
dorsal to the common iliac arteries. Thus under Type II, in which the 
union between the internal and external iliac veins takes place exclusively 
dorsal to the arteries, twenty cases were met with, and under Type III, B, 
in which the anastomosis ventral to the arteries is insignificant in char- 
acter, twelve cases were met with in which the postcava was either bifur- 
cated or presented some indication of a previous bifurcation. Under 
Type III, C, in which the vessel which lies ventral does not exceed in 
size that which lies dorsal to the iliac arteries, six cases were met with in 
which the postcaval veins were incompletely fused caudal to the internal] 
spermatic veins. 
Under Types I and III, A, in which the posteava is formed either 
