280 THE ANATOMY OF YERTEBRATED ANIMALS. 



There is a great range of variation in tlie characters of tho 

 brain. The carnivorous Marsupials {Didelphys, Dasyui'iis^ 

 Thylacinus) exhibit the lowest type of cerebral structure, the 

 olfactory lobes being very large and completely exposed, 

 while the cerebral hemispheres are comparatively 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 olfactory lobes, w^hich they 

 cover. 



The stomach may be simple, as in most 3Iar8upialia^ or 

 provided with a cardiac gland {Phascolarctos, JPhascolomys). 

 In the Kangaroos, it becomes immensely elongated, with 

 longitudinal muscular bands and transverse sacculations, so that 

 it resembles the human colon. The caecum, which is large in 

 the Kangaroos, but absent in the Dasyuridoe^ is provided, in 

 the Wombat, with a vermiform appendix like that of Man. 



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

 '^enrje cavcB superiores, and they receive the vence azyc/os of 

 their respectiv^e 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 vesiculge seminales, and the glans penis is 

 bifurcated in many species. The marsupial pouch is absent iu 

 some Opossums and Dasyuridm. When it is present, its 

 mouth is usually directed forward, but in Thylacinus and in 

 some Peratnelidm it looks backward. In Thylacinus also the 

 " marsupial bones " remain cartilaginous. The condition of 

 the foetus is known only in the Kangaroos, and further observa- 

 tions on the embryology of the Didelphia are much needed. 

 The foetus is said to possess a large umbilical sac, the vessels 

 of which extend on to the plaited chorion ; and a small allan- 

 tois ; and to be devoid of a thymus gland. 



The Didelphia are at present confined to the Australian 

 and the Austro-Columbian provinces, some few species stretch- 

 ing beyond the borders of the latter into the northern parts of 

 North America. The Didelphidm alone are found in Austro- 

 Oolumbia, all the other groups being Australian. 



Gigantic, Kangaroo-like, or Phalangistic, forms {^N'otothe- 

 rium^ Diprotodon^ Thylacoleo), have been found in post-ter- 

 tiary deposits and caves in Australia. In Europe, DldelphidcB 

 occur in Eocene strata ; Didelphidce, Dasyuridm^ and Macro- 

 podidoe {Phascolotherium^ Amphitherium^ Plagiaiilax)^ in 

 middle Mesozoic rocks ; and Macropodidm (?) {JSIicrolestes) in 

 the Trias. 



