Charles F. W. McClure . 393 
means of a single vessel (V. iliaca interna communis) into either the left 
(Fig. 9) or into the right (Fig. 10) external iliac vein. In the adult, 
an extraordinary similarity exists between the posteava in Hchidna 
aculeata and Type II in Didelphys. Compare Hochstetter’s Fig. 16 
(96, Taf. 28) of this vein in Hchidna with my Fig. 6 of Didelphys. 
Type I11—This Type is a combintion of Types I and IT, since 
it includes those cases in which the Vv. iliace interne unite 
with the Vv. iliace externe to form the postcava, both dorsgl 
and ventral to the Aa. iliace communes or both dorsal and ventral 
to the aorta. 
Including the three subdivisions, A, B and C, forty-three examples of 
this Type were met with distributed among fourteen males and twenty- 
nine females. 
As shown by the above table, the representatives of this Type constitute 
about 43 per cent of the variations observed, which is almost twice that 
of either of the other Types. Considering the composite character of 
Type III, however, it is evident that this predominance possesses no 
particular significance beyond the fact that the veins that lie dorsal and 
ventral to the umbilical arteries in the embryo, both possess a marked 
tendency to persist in the adult. 
Type III, A—tIncludes those cases in which the principal 
union between the Vy. iliace interne and externe takes place 
ventral to the arteries in question. 
Eight examples of Type III, A, were met with distributed among three 
males and five females. . 
See Figs. 11, 12 and 13 (dorsal views) and Fig. 14 (ventral view of 
Hig 13); Plate Gir 
Two sets of variations within this Type were met with: 
First, as in Figs. 11 and 12 (dorsal), in which the internal iliac veins 
anastomose with the external iliac veins ventral to the arteries, as in 
Type I (Fig. 1), and, in addition to this, by means of a single small 
vessel which les dorsal to the left common iliac artery. In one indi- 
vidual the caudal vein opened into this dorsal anastomosis (Fig. 12, 
dorsal view). 
Second, as in Figs. 13 and 14, in which the internal iliac veins 
anastomose ventral to the A. sacralis media (Fig. 14) and then join the 
external iliac veins to form the postcava by means of fowr vessels, two 
of which, the largest, lie ventral (Fig. 14), while the other two lie dorsal 
(Fig. 13) to the common iliac arteries. 
Type IfI, B, includes those cases in which the principal 
, 
27 
