Comparative Anatomy of the Brain. 585 



Dasyure, a marsupial, shows the olfactory and optic lobes exposed from 

 above as well as the cerebellum. In these brains there is no fully de- 

 veloped corpus callosum. The cerebral hemispheres are united by a 

 round commissure, and by a ' lyra ' and hippocampal commissure. 

 This appears to be the forerunner of the corpus callosum, which is de- 

 veloped in the higher mammals. In the monotremes the optic lobes are 

 still a single pair, as in birds. 



Owen's Classification of Mammals based on Brain Development. Class Mammalia. 



The characteristics that go with monotremes and marsupials connect 

 them with the birds and reptiles on one side, and the mammals on the 

 other. As shown in chapter 4, they are non-placental, a circumstance 

 in which they are unlike all other mammals. The young are brought 

 forth in a very immature state. The kangaroo mother conveys her 

 young to her pouch with her lips ; after a uterine period of 38 days 

 they attach themselves to the teats, and there remain for eight months. 

 The monotremes having no pouch leave their young in a nest, like the 

 young of rats, &c. A young ornithorhynchus is from one to two 

 inches long. Its tongue and lungs are well developed. But it is blind, 

 and its horny bill is not developed, obviously to its advantage while it 

 has to suck, especially as the mother has no projecting teats. 



