Charles F. W. McClure 385 
A connection between the azygos vein and the postcava is apparently 
not of constant occurrence in the marsupials. Beddard, 95, found 
such a connection invariably occurring in Didelphys azare. In Phalan- 
gista, however, he found it occurring in only one of several individuals 
examined and, in this case, on account of its large size, it practically 
took the place of the postcava. In Didelphys the writer has never 
observed any diminution in size of the postcava on account of its con- 
nection with the azygos vein. 
There is very little in the literature that refers to the manner in 
which the blood is collected from the intercostal spaces in marsupials. 
Forbes, 81, states that when two azygos veins are present the smaller 
vein collects blood from the first few intercostal spaces (Cuscus, Beli- 
deus, Phascolomys and Phalangista). According to Cunningham, 82, 
the azygos tributaries collect blood in Thylacinus from the intercostal 
spaces of both sides with the exception of the first three; the inter- 
costal veins of these three spaces join, on each side, to form a single 
vessel which opens into the precava. In Cuscus, Phalangista vulpina, 
Phascogale and Dasyurus the single left azygos vein collects blood from 
all of the intercostal spaces on the left side and from all on the right 
side except the first three; these, as in Thylacinus, return their blood 
through a single vessel that opens into right precava (Cunningham). 
In Petrogale xanthopus the azygos vein receives all of the intercostal 
veins from both sides of the thorax (Parsons, 96). 
In a specimen of Petrogale recently dissected by the writer, although 
the vessels in this region were poorly injected, the blood from all of 
the intercostal spaces on the right side appeared to be collected by 
tributaries of the right azygos. On the left side the intercostal veins 
from all the intercostal spaces except the first two or three also opened 
into the right azygos; the veins from the first two or three spaces 
joined, as on the left side in Thylacinus, to form a single vessel which 
opened into the left precava. The small left azygos vein, which was 
present in addition to the left superior intercostal, was not injected, so 
that its connections could not be traced. A V. costovertebralis 
which has been described above as a constant tributary of 
the precava in Didelphys was wanting on both sides in Petrogale. 
It is evident from what has been stated above that there is little 
agreement between Didelphys and other marsupials so far as the veins 
of the first three or four intercostal spaces are concerned. Some of 
these differences can undoubtedly be explained on the ground that 
they are individual variations. The most interesting and fundamental 
difference, however, is the circumstance that in Didelphys the most 
