300 Illinois State Lahoratory of Natural History. 



line is roughly indicated in alcoholic specimens by col- 

 lapse of its dorsal wall, which usually gives rise to a 

 depressed area on the posterior dorsal surface of the fore- 

 braiu. In front of the dorsal sac the dorsal wall of the 

 fore-brain, viewed from within, presents a slight median 

 ridge, while anteriorl3% between the olfactory lobes, is a 

 slight membranous fold, these being the only indication 

 of separation of the cavity of the fore-brain into two 

 ventricles. The fore-brain of A. rvhicundus shows a sim- 

 ilar but, I think, somewhat larger trace of a septum 

 between the halves of this division of the brain. The 

 lateral wall of the fore-brain of Polyodon is provid- 

 ed within with thin vertical plicae, resembling those on 

 the inside of the dorsal membrane of the fourth ventricle. 



Arising from the dorsal wall of the brain at the junc- 

 tion of the fore- with the mid-brain is a slender white 

 filament having the appearance, to ordinary inspection, 

 of a nerve. At first it is concealed by the dorsal wall of 

 the dorsal sac. By reflecting this wall it appears run- 

 ning along the ventral side of the wall as a whitish line. 

 It emerges to the exterior near the anterior extremity 

 of the dorsal sac and extends thence obliquely upward 

 and forward to the cartilaginous cranium, where it passes 

 into a special channel, along the middle of which it ex- 

 tends in a direct course nearly to the surface. In a fish 

 which must have been about 3^ feet long, the end organ 

 to which this filament passes lies about 1.66 mm. from 

 the outer surface. The stalk expands as it enters the 

 end organ. It is enclosed throughout its course in a 

 membranous sheath, which gives it an appearance of 

 being larger than it really is. It is accompanied by small 

 blood vessels which supply the walls of the passage and 

 eventually give small capillary vessels to the end organ. 



The end organ is also enclosed in a sheath continuous 

 with that surrounding the stalk. From the surface of 

 the sheath radiate numerous small slender fibers (mus- 

 cular?) which pass to the walls of the cavity in which 



