4 Transactions of the 



structure of these two layers, so essentially diflferent, I will at once 

 describe the results of my investigation. 



The inner layer, or rather each inner layer, as there are two, 

 is divided into well-marked hexagonal areolae, each hexagon having 

 a central cii'cular spot, which gives, both in air and bisulphide of 

 carbon, a positive image when beyond the focus, proving that it has 

 neither a spherical nor concave form. 



If it were convex it would, as previously shown, give a negative 

 image in bisulphide ; if it were concave it would give a negative 

 image in air ; but giving, as it does, a positive image in both, it 

 follows that it must be either a perforation, or, if not a perforation, 

 it is occupied by a plate of silex which gives no lens-like action in 

 any medium. 



That these spots are, however, absolutely openings, there can, 

 I think, be little doubt ; as in no case can I detect, with any power 

 up to a Powell and Lealand's ^V? ^'^1 trace of a broken film, 

 although I have broken the layer in all directions, and the line 

 of fracture almost invariably passes through some of the circular 

 markings. Fig. 3. 



As the fact of their being open or closed may involve a ques- 

 tion of considerable physiological interest, it is well to compare 

 them with the foramina of the silicious skeletons of the Poly- 

 cistinae — known to be foramina because they give egress to the 

 characteristic pseudopodia of these animals — by so adajiting the 

 power employed that the openings may appear of about the same 

 magnitude in each case. 



In making the comparison it is well to select the smaller, 

 broken polycistins, when it will be seen that the optical appear- 

 ances presented are strikmgly similar in the two cases, and strongly 

 support the view which I have ventured to enunciate. 



These circular openings, as I will venture to call them, in the 

 inner plate, are bounded by a thicker ring of silex, and the several 

 hexagonal areolae are also divided from each other by similar bands, 

 as indicated by their becoming black when beyond the focus, and 

 bright within, when mounted in bisulphide. Figs. 2 and 4. 



The outer layer is more complex in its structure, and many times 

 thicker than that just described. It is formed of deep hexagonal 

 cells, the depth of each cell being, as nearly as I could determine 

 from a side view of a small fragment, about one and a half times 

 the diameter ; but this probably varies in diflferent parts of the disk ; 

 these also give, when beyond the focus, positive images in each 

 medium — proving, as in the case of the inner plate, that these cells 

 are either open at each end, or, if closed, that they are so by a film 

 or plate which is not of a single lens-like form. Figs. 5 and 6. 



Inside and around each cell is a beautiful ring of bright spots, 

 about sixteen in number, which, if seen in air, would at once be 



