Charles F. W. McClure - Paha 
similar questions cannot be definitely decided without examining a large 
number of embryos and pouch young; and, furthermore, without, at the 
same time being fortunate enough to meet with a similar type of varia- 
tion. Whatever the case may be regarding the origin of these ventrally 
situated vessels, the venous channels which can be retained in the adult 
are all well defined in the embryos and pouch young. And, although 
certain difficulties are apparent in determining, in all cases, which of 
these embryonic venous channels have actually been retained in the 
adult, it is not impossible to interpret the adult variations (three types 
of posteaval veins) on the basis that they represent the possible combina- 
tions which could ensue as the result of the persistence or atrophy of 
certain of these embryonic vessels. 
EXPLANATION OF THE Two ABNORMALITIES OF THE PostcaAvaA WHICH 
CANNOT BE CLASSED UNDER TyPss I, II anp III (sEE 
Text Figs. VII ann VIII, Part [). 
As stated on page 395, Part I of this paper, the main features 
which characterize these two abnormalities and distinguish them from 
the variations described under the three types are twofold: (1) All of 
the posterior tributaries of the postcava, including the external or com- 
mon iliac veins, as the case may be, unite dorsal to the arteries to form 
the postcava, as is usually the case in placental mammals; (2) the post- 
cava lies to the left of, instead of upon the ventral surface of, the aorta, 
and resembles in this respect the conditions met with in placentals when 
the left instead of the right postcardinal vein persists as the caudal end 
of the postcava. 
Both of these variations are figured on page 396, Part I of this paper 
(Figs. VII and VIII) to which the reader is referred. 
It appears to the writer that these two abnormalities in question may 
be explained on the ground that the caudal portion of the unpaired post- 
cava has been formed, in each case, from one (the left) instead of from 
both of the cardinal collateral veins, as is usually the case in Didelphys. 
The persistence of the left vein explains the position of the postcava on 
the left side of the aorta; while the persistence of a single vein instead 
of two to form the caudal section of the postcava possibly duplicates the 
same physiological conditions that prevail in most placental mammals, 
and necessitates a union between the postcava and the iliac tributaries of 
the opposite side dorsal to the arteries. 
‘The position of the ureters was normal in both cases since they were 
situated lateral to the postcava along their entire extent. For this reason 
