72 Journal of Comparative Neurology. 



the lamina terminalis. The chief effect of the pressure has 

 been to produce a lateral bulging, and in this the cavity has 

 participated. The form of the lateral cavities is readily ex- 

 plained by this alone. The cephalic horn is the cavity of the 

 olfactory lobe in its usual relations ; the caudal horn is the cav- 

 ity of the fore brain carried outward and backward by the bend- 

 ing of its wall. The lateral bulging of the wall has resulted in 

 a partial separation of the striatum and epistriatum, the epistri- 

 atum extending forward above the lateral cavity. 



Houser notes the presence of spiny gemmules on the den- 

 drites of striatum cells in Mustelus and suggests that they are 

 characteristic of these cells in fishes, citing van Gehuchten 

 ('94) and Johnston ('98 a) in evidence. In my later paper ('01 c) 

 it was stated that the striatum cells are usually devoid of these 

 gemmules and that they are characteristic of the epistriatum 

 cells. I am convinced that it is exceptional for the striatum 

 cells to bear spines in Acipenser and they never do in Petro- 

 myzon. It should be noticed further that van Gehuchten 

 probably saw only the epistriatum cells and erroneously sup- 

 posed that their neurites joined the basal bundle. 



c. The 'tween brain. 



The comparative simplicity of the 'tween brain is one of 

 the most marked characteristics of the brain of Petromyzon. 

 Although the ganglia habenulae and epiphysis are large and 

 apparently more important than in Acipenser, the remainder of 

 the 'tween brain shows less differentiation. The nucleus ante- 

 rior is not to be recognized and the central grey is more com- 

 pletely a nucleus diffusus than in other fishes, while in the 

 hypothalamus the inferior lobes and corpus mammillare show 

 too little differentiatio;i to be considered separate centers. A 

 large part of the cells of the central grey of the thalamus and 

 mid brain send their neurites through the posterior commissure 

 and constitute a nucleus homologous with that described by 

 KoLLiKER in mammals. The saccus corresponds in structure 

 and relations so far as studied, with that of Acipenser. 



d. The tectum. 



The tectum has been shown to be more simple than In 



