302 lllinou State Laboratory of Natural History. 



to this point, two other examples dissected, but I am 

 disjiosed to think such passage could not have been pres- 

 ent in them, else it would have caught my attention. 



From the outside the position of the end organ can be 

 made out at once in Polj'odon bj- the presence of an 

 elliptical foramen* in the bony exoskeleton which covers 

 the cartilage of the head. It shows as a depressed area 

 when undisturbed, and is occupied by skin in which one 

 can see with a magnifier uniformly distributed pigment 

 specks. In one of my specimens the foramen measures 

 7 mm. in diameter and 19 mm. in length. It extends 

 lengthwise with the cranium, and lies between the eyes. 

 I can see no change in the character of the skin imme- 

 diately over the end organ, which lies beneath or a trifle 

 posterior to the center of the foramen. When the skin 

 occupying the foramen is removed the cartilage under 

 it is seen to be gently convex; and when the surround- 

 ing bony exoskeleton is also taken away the region is 

 still perceptible, owing to this convexity and to the dif- 

 ferent color and greater translucency of its cartilage, 

 through which, by close looking, one can see the end 

 organ. The adjoining cartilage is of a more opaque 

 whitish cast. The thickness of the cartilage and skin 

 together just above the end organ measures in one of my 

 examples 1.75 mm., of which each tissue constitutes 

 about one half. The end organ in this specimen lies 0.50 

 mm. beneath the cartilage, and measures about one 

 millimeter in diameter, excluding the enclosing sheath. 



In Acipenser ruhiciindus the pineal structures are in gen- 

 eral much like those of Polyodon. In this fish the stalk 

 is concealed at first beneath the wall of the dorsal sac and 

 reaches the roof of the brain proper at the same point 

 as in Polyodon. It is accompanied into the channel in 



♦The foramen m completely surrounded by the "frontal" bones of Collinge 

 'Quar. Jour. Mier. Science, Vol. 3«. 1894. pp 49<)-K57). Thes« bones are not sep- 

 arate at the middle line, as represented by this author, but moet both in 

 front and behind the foramen, so that the median bone, the "dermo- 

 ethmoid," is excluded from the foramen in front. Collinge may have described 

 the bones of an immature example; othorwiso the difference between his ac- 

 count and my own indicates considerable variation. 



