Charles F. W. McClure 
397 
presence of a postcava in Petawrus taguanoides which is similar to that 
of a higher mammal, as well as for the presence of an allantoic placenta 
in Perameles. 
In addition to the above, another abnormality was met with which, 
although relating to the arterial system, is quite as remarkable. 
As is well known, the posterior mes- 
enteric artery is wanting in all marsu- 
pials which have thus far been exam- 
ined, its place being taken by a branch 
of the anterior mesenteric artery (Pet- 
rogale) or by a branch of the A. ccelia- 
comesenterica (Didelphys) as the case 
may be. Among the one hundred 
and one opossums examined by 
the writer one was met with in 
which a large posterior mesen- 
teric artery was present, which 
took the place of the usual pos- 
terior mesenteric branch of the A. 
celiacomesenterica. In this animal 
the posterior mesenteric artery (A. m. 
p-) was given off from the caudal end of 
the aorta and passed ventrad through a 
foramen (F.) in the postcaval vein 
(Text Fig. IX). In its point of origin 
trom the aorta as well as in its distri- 
bution to the intestines this artery 
agreed in all respects with the posterior 
mesenteric arteries of other mammals. 
In addition to its branches to the intes- 
tines it also gave off as branches, after 
passing through the foramen, the two 
posterior internal spermatic arteries 
(A. s. i. p.) which were distributed to 
the ovaries in the usual manner.” 
Fie. IX. Postrenal division of post- 
eava of Didelphys marsupialis in which 
a foramen is present through which a 
posterior mesenteric artery passes. 
A.ie., A. iliaca externa; A.m.p., A. 
mmesenterica posterior; Ao., Aorta; 
A.s.i.a., A. spermatica interna anterior ; 
A.s.i.p., A.spermatica interna posterior; 
F., foramen; L.G., lymph gland; pe., 
posteava; V.i.e., V. iliaca externa; V.i.i., 
V.iliaca interna; V. il., V. iliolumbalis; 
V.r.,V. renalis; V. sp.i., V. spermatica 
interna. 
Owen, 66, has stated that the absence of the posterior mesenteric artery 
in marsupials is probably related to the simplicity of the mesenteric 
19Tn a preliminary paper, oo, the writer mentioned the circumstance that two pairs 
of internal spermatic arteries may spring from the aorta in Didelphys. 
It may be 
stated here that these two pairs of arteries occur with great regularity in Didelphys 
and that they were also present in the specimen of Petrogale recently dissected by the 
