24 



concerned in the nourishment of the progeny before birth, 

 called t placenta;' the young in all this ' implacental ' divi- 

 sion being brought forth prematurely, as compared with the 

 rest of the class. 



This first and lowest primary group, or subclass, of Mam- 

 malia is termed, from its cerebral character, LYENCEPHALA I , 

 signifying the comparatively loose or disconnected state of 

 the cerebral hemispheres. The size of these hemispheres 

 (fig. 4, A) is so small that they leave exposed the olfactory gan- 

 glions (a), the cerebellum (c), and more or less of the optic 

 lobes (B) ; their surface is generally smooth ; the anfractuosi- 

 ties, when present, are few and simple. 



Fig- 5- 



Fig. 4 . 



Brain of Opossum. 



Brain of Beaver. 



The next well marked stage in the development of the 

 brain is where the corpus callosum (indicated in fig. 5, by the 

 dotted lines d, d) is present, but connects cerebral hemispheres 

 as little advanced in bulk or outward character as in the pre- 

 ceding subclass ; the cerebrum (A) leaving both the olfactory 

 lobes (a) and cerebellum (c) exposed, and being commonly 



tfw, to loose ; 



, brain. 



