390 Venous System of Didelphys Marsupialis 
case is quite different. Here instead of occurring as exceptions 
the variations appear to be the rule, so that it is actually 
impossible in this animal to assign any one mode of origin for 
the postecava that may be regarded as characteristic of the 
species. ‘This opinion was first advanced by the writer in a preliminary 
article in 1900, but on the basis of less extensive observations. 
For descriptive purposes the various modes of origin of the postcava 
in Didelphys marsupialis have been classified by the writer under three 
Types as follows: 
Type I—Those cases in which the Vv. iliace interne unite 
with the Vv. iliace externe to form the postcava, ventral to the 
Aa. iliace communes or ventral to the aorta. 
Type II.—Those cases in which the Vv. iliace interne unite 
with the Vv. iliace externe to form the postcava, dorsal to the 
Aa. iliace communes or dorsal to the aorta. 
Type IIJ.—Those cases in which the Vv. iliace interne unite 
with the Vv. iliace externe to form the postcava, both dorsal 
and ventral to the Aa. iliace communes or both dorsal and ventral 
to the aorta. 
So many variations of this last 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 Vv. iliace interne and externe takes 
place ventral to the arteries in question. 
Type III, B.—Includes those cases in which the principal 
union between the Vv. iliace interne and externe takes 
place dorsal to the arteries in question. 
Type III, C—Includes those cases in which the above men- 
tioned dorsal and ventral unions are about subequally developed. 
One hundred and one (101) opossums were examined and, in all but 
two, the different variations presented by their postcaval veins 
could be classed under the above mentioned three Types. In 
these two individuals, however, the postecava neither in its posi- 
tion, with respect to the aorta, nor in its mode of formation, 
conformed to the marsupial type but rather to the type of 
posteava which is characteristic of the higher mammals. 
The following table shows the distribution of the above mentioned 
‘Types among ninety-nine individuals (34 males and 65 females) : 
1s When the postcava is bifurcated, in this as well asin the following Types, it is 
the common iliac veins that are formed instead of the postcava. 
