THE DIDELPHIA. 327 



fourth toe is very large. The hallux is reduced to its 

 metatarsal bone in the Peramelidoe, and the fifth digit is 

 small or rudimentaiy. In the Kangaroos, the hallux dis- 

 appears altogether, but the fifth digit remains well de- 

 veloped, though not so large as the fourth. 



There is a great range of variation in the characters 

 of the brain. The cai-nivorous marsupials (Bidelphys, 

 Dasyurus, Thylacinus) exhibit the lowest type of cerebral 

 structure, the olfactory lobes being very large and com- 

 pletely exposed, while the cerebral hemispheres are com- 

 paratively small and quite smooth. In the Kangaroos, on 

 the other hand, the cerebral hemispheres present numerous 

 convolutions and are much larger in proportion to the 

 oKactory lobes, which they cover. 



The stomach may be simple, as in most Marsupialia, or 

 provided with a cardiac gland {Phascolarctos, P.hascolomys). 

 In the Kangaroos, it becomes immensely elongated, with 

 lons-itudinal muscular bands and transverse sacculations, 

 so that it resembles the human colon. The coecum, which is 

 large in the Kangaroos, but absent in the Dasyuridce, is 

 provided, in the Wombat, with a vermiform appendix like 

 that of Man. 



The Uver always possesses a gall-bladder. There are two 

 vence cavoe superiores, and they receive the vencB azygos of 

 their respective sides. The tricuspid valve in the heart is 

 membranous. There is no inferior mesenteric artery, and 

 the external and internal iliacs arise separately from the 

 aorta. 



There are no vesiculee seminales, and the glans penis is 

 bifurcated in many species. The marsupial pouch is absent 

 in some Opossums and Dasyuridce. When it is present, its 

 mouth is usually directed forwards, but in Thylacinus and 

 in some Peramelidce it looks backwards. In Tliylacinus also 

 the " marsupial bones " remain cartilaginous. The con- 

 dition of the foetus is known only in the Kangaroos, and 

 farther observations on the embryology of the Didelphia 

 are much needed. The fcetus is said to possess a large 

 umbilical sac, the vessels of which extend on to the plaited 



