BRAIN. 



99 



ing, when present, the boundary between prosencephalon and 

 mesencephalon. The lobi optici are essentially as in Tele- 

 osteans. The cerebellum penetrates into the ventriculus 

 lobi optici, and extends thence into the open sinus rhomboid- 

 alis. At its upper surface it is crossed by a commissure 

 formed by the coiyora restiformia of the medulla. 



A 



Fig. 42.— Brain of Polypterus. (After Mliller.) 

 I., Upper; II., Lateral ; III., Lower aspect. 

 a, Medulla ; h, corpora restiformia ; c, cerebelhim ; d, lobi optici ; e, hypophysis ; 

 /, fissura cerebri magna ; g, nervus opticus ; g', chiasma ; h, hemispheres ; 

 i, lobus olfactorius ; k, sinus rhomboidalis (fourth ventricle). 



As regards external configuration, the brain of Zepidosteus 

 and Amia approach still more the Teleosteous type. The 

 prosencephalon, mesencephalon, and metencephalon are con- 

 tiguous, and the cerebellum lacks the prominent transverse 

 commissure at its upper surface. The sinus rhomboidalis is 

 open. 



The brain of the Dipnoi shows characters reminding us 

 of that of the Ganoids as well as the Chondropterygians, 

 Ceratodus agreeing with Protopterus in this respect, as m 



