200 Venous System of Didelphys Marsupialis (L) 
dividuals, however, neither in its position with respect to the aorta nor 
in its mode of formation through a union of the iliac veins does the 
postcava conform to the usual marsupial type, but rather to the type of 
postcava which is characteristic of most placental mammals and, for this 
reason, these two exceptions are regarded by the writer as the only cases 
of postcaval abnormalities met with among the 101 opossums examined 
@Pare EL p..595): 
The three types under which the postcaval variations are classified are 
as follows: 
Type I. Those cases in which the internal iliac veins unite with the 
external iliacs ventral to the common iliac arteries or ventral to the 
aorta to form the postcava. 
This type of postcava is the one commonly met with among the Aus- 
tralian marsupials, and may be spoken of as the marsupial type (see 
Text Fig. VI, Part I). The writer at present knows of but three cases 
among the Australian marsupials, thus far examined, in which the post- 
cava is formed in any other manner; two in which i¢ is formed as in 
placentals and in a manner similar to that in the cat (Petaurus tagua- 
noides” and Phalanger ursinus”) and one, Trichosurus vulpecula,” 
in which it is formed as in Didelphys (Type IL) and as figured on Plate 
ie wie: 6, Parte): 
Type II. Those cases in which the internal iliac veins unite with 
the external iliacs dorsal to the external iliac arteries, or dorsal to the 
aorta to form the postcava. 
Type III. Those cases in which the internal iliac veins unite with 
the external iliacs both dorsal and ventral to the common iliac arteries 
or both dorsal and ventral to the aorta to form the postcava. 
So many variations of this last type were met with that a further sub- 
division of Type III was found necessary, as follows: é 
Type III, A. Includes those cases in which the principal union be- 
tween the internal and external iliac veins takes place ventral to the 
arteries In question. 
Type III, B. Includes those cases in which the principal union be- 
tween the internal and external iliac veins takes place dorsal to the 
arteries in question. 
Type III, C. Includes those cases in which the above-mentioned 
dorsal and ventral unions are about subequally developed. 
* Hochstetter, 93. 
1 Morphological Museum, Columbia University, No. 199. 
@ Morphological Museum, Columbia University, No. 234. 
