394 Venous System of Didelphys Marsupvalis 
union between the Vv. iliace interne and externe takes place 
dorsal to the arteries in question. 
Twenty-four examples of Type Ill, B, were met with distributed 
among nine males and fifteen females. 
See Figs. 15, 16, 17, 18, 19, 20, 21, and 22 (ventral views), Plate IV. 
Three sets of variations were met with within this Type: 
First, as in Figs. 15, 16, 17, 18, and 19, in which the internal iliac 
veins unite with the external iliac veins to form the postcava (in this 
case the common iliac veins), by means of three vessels, two of which, 
the largest, lie dorsal, and one, the smallest, lies ventral to 
the common iliac arteries. 
In Figs. 15 and 16 the internal iliac veins do not anastomose ventral 
to the A. sacralis media, and the small vessel which lies ventral to the 
common iliac artery extends between the right (Fig. 15) or left (Fig. 16) 
internal iliac vein and the common iliac vein of the opposite side. In 
Fig. 17 the postcava is formed in essentially the same manner as in Figs. 
15 and 16, with the exception that the small vessel which lies ventral to 
the common iliac artery extends, in Fig. 17, between the iliac veins of 
the same instead of opposite sides. This is a most interesting varia- 
tion since it represents the persistence, on one side, of the 
venous ring through which the umbilical artery passes in the 
embryo (McClure, 02). In Fig. 18 the internal iliac veins anastomose 
ventral to the A. sacralis media, but in every other respect this variation 
is similar to that represented by Fig. 16. In Fig. 19 the conditions are 
the same as in Fig. 18 except that in Fig. 19 the small vessel which les 
ventral to the common iliac artery has changed its position with respect 
to the iliac veins as the result of a fusion of the common iliac veins. 
Second, as in Fig. 20, in which the internal iliac veins anastomose 
ventral to the A. sacralis media and then unite with the external iliac 
veins to form the postcava by means of three vessels, two of which, one 
jarge and one small, lie dorsal, while one, quite large, lies 
ventral to the common iliac arteries. This variation differs from 
the first set only in the circumstance that the single vessel which lies 
ventral to the common iliac artery is as large as the largest of the two 
vessels which lie dorsal to this artery. 
Third, as in Figs. 21 and 22, in which the internal iliac veins anasto- 
mose ventral to the A. sacralis media and then join the external iliac 
veins to form the postcava (or common iliac veins) by means of two 
vessels. The larger of these two vessels, which lies dorsal to the common 
iliac artery, may join the right (Fig. 21) or left (Fig. 22) external iliac 
vein, while the smaller vessel, which lies ventral to the common iliac 
artery, then joins the external iliac vein of the opposite side. 
