Brain and Pineal Structures of Polyodon folium. 299 



walls of the olfactory lobe, into two divisions, with 

 some slight intermingling of fibers at the base. The 

 divisions lie parallel at first, but separate completely 

 at about the middle of their course, and before reach- 

 ing the nasal sac each divides into a number (seven 

 were counted) of small diverging branches passing into 

 the walls of the sac. One can see in the olfactory nerve 

 of Acipenser rubicundus an indication of doubleness, but 

 in Polyodon it is carried much further, the divisions 

 being independent except for the slender fibers which 

 cross from one to the other at the base. The nerves 

 have a very slight upward turn as they pass out to the 

 sac. 



The olfactory lobe is well developed in both Polyodon 

 and A. rubicundus, but is not a conspicuous part of the 

 brain in either fish. To ordinary inspection it looks 

 like a slight enlargement of the base of the olfactory 

 nerve. It contains a ventricle of large relative size 

 which communicates with the ventricle of the fore-brain. 



The fore-brain presents no outward trace of double- 

 ness. When viewed from above it is oval in general 

 shape, widest at about the middle, and is obtusely 

 angulate medially in front. Viewed from the side it is 

 seen to rise above the level of the olfactory lobes. It 

 appears not to be so well developed in Polyodon as in 

 Acipenser. The division commonly termed the thala- 

 mencephalon can hardly be said to exist, though the 

 coustrictiou of the brain at this region and the' positions 

 of the epiphysis and hypophysis definitely locate it. 

 The ventral wall of the fore-brain bears two large, lobed, 

 white, basal ganglia which encroach a good deal upon 

 the ventricle. The front extremity of each lies opposite 

 the opening into the ventricle of the olfactory lobe. A 

 membranous dorsal sac is present which communicates 

 freely posteriorly with the cavity of the fore-brain. It 

 is not as conspicuous in Polyodon as in our sturgeon, 

 and is not perceptible from without except as its out- 



