25 



smooth, or with few and simple convolutions in a very small 

 proportion, composed of the largest members, of the group. 

 The mammals so characterized constitute the subclass LISSEN- 



CEPHALA* (fig. 5). 



The third leading modification of the Mammalian cere- 

 brum is such an increase in its relative size, that it extends 

 over more or less of the cerebellum ; and generally more or 

 less over the olfactory lobes. Save in very few exceptional 

 cases of the smaller and inferior forms of Quadrumana (fig. 6), 

 the superficies is folded into more or less numerous gyri or 

 convolutions (fig. 7), whence the name GYRENCEPHALA, which 

 I propose for the third subclass of Mammalia 2 . 



Fig. 7. 



Fig. 6. 



Brain of Marmoset Monkey. 

 (Nat. size). 



Brain of Chimpanzee. 

 (Half nat. size). 



In Man the brain presents an ascensive step in develop- 

 ment, higher and more strongly marked than that by which 

 the preceding subclass was distinguished from the one below 

 it. Not only do the cerebral hemispheres overlap the olfac- 

 tory lobes and cerebellum, but they extend in advance of the 



1 \i<7<rds, smooth ; tyiftyaXos, brain. 



2 "yvpbii), to wind about ; eyK^(f>a\os, brain. 



