Charles F. W. McClure 199 
way between the left renal and common iliac veins (Fig. 28, Plate I) ; 
and, when the posteava was bifurcated, the connection was invariably 
found at the level of the bifurcation (see Part I, Fig. 8, Plate IT). In 
none of the adults examined did the spermatics open into the renals, al- 
though an anastomosis between the latter and the spermatics was in- 
variably present, on each side, in the form of a small vein which fol- 
lowed the ureter (see Fig. 28, Plate I). 
In a number of adult Australian marsupials, however, the spermatics 
do not open into the postcava, as in Didelphys, but open into it at the 
base of the renal veins, as in Phascolomys Mitchelli (McClure, 03, p. 
388), or into the renal veins themselves, as in Notoryctes typhlops 
(Sweet, 04). Considering the position at which the spermatic veins are 
developed in Didelphys, the question arises, how can these differences 
be explained? At the present time but two possibilities suggest them- 
selves to the writer: (1) Either the lumbar portion of the postcava may 
not elongate to such an extent in these two forms as in Didelphys, so 
that the renal veins are developed, as in the rabbit and cat, at the level 
of the root of the posteava (anastomosis between postcava and _ post- 
cardinals, Text. Fig. 7); or, (2) if an elongation does take place, as 
in Didelphys, the connection of the spermatics with the renal veins in 
Notoryctes may be accounted for on the ground that the spermatics have 
given up their original connection with the root of the postcava in favor 
of the channel, mentioned above, which follows the ureter and which 
opens into the renal veins. 
THE VARIATIONS PRESENTED BY THE POSTCAVA IN 
THE ADULT DIDELPHYS. 
In all mammals, hitherto examined, the posteava is formed in the 
adult through a union of its iliac tributaries which takes place in a defi- 
nite and uniform manner so that when variations occur they are re- 
garded as exceptions to the general rule. Such, however, is not the case 
in Didelphys marsupialis. Here, instead of occurring as exceptions, va- 
riations appear to be the rule, so that it is actually impossible in this 
mammal to assign any one mode of origin for the postcava that may be 
regarded as typical of the species. This opinion is based upon the ex- 
amination of 101 individuals; and a full description of these variations, 
as well as figures of the same, have already been published in Part I of 
this paper, to which the reader is referred (page 390). 
In all but two of the 101 adult opossums examined the variations of 
the posteava can be easily classed under three main types. In two in- 
