N. E. BROWN ON THE STRUCTURE OF DIATOMS. 331 



grating shows a somewhat bead-like appearance, a very minute 

 dusky dot, which I take to be a pore, is just discernible in the 

 centre of every one of them, as represented in fig. 12. These 

 pores, I think, are smaller even than those of Sarirella gemma, 

 and are very difficult to see, unless perhaps to younger eyes, as 

 I judge them to be about the limit of my vision. At a slightly 

 lower focus the nodes formed by the junctions of the transverse 

 and longitudinal bars assume the well-known black-dot appear- 

 ance, and all trace of the other structure disappears. Doubtless 

 the structure of A. pellucida is similar. 



Coscinodiscus heliozoides. I have nothing to remark 

 upon the structure of the diatom to which Mr. Siddall recently 

 gave the above name; but I should like to call the attention of 

 experts to its remarkable similarity to Stepkanodiscus Hantz- 

 schianus. I have not been able to compare the two, but feel 

 sure that C. heliozoides belongs to the genus Stephanodisats, 

 and have a suspicion that it and S. Ilantzschianus are one 

 and the same diatom. A good figure of the latter will be found 

 in the Deutschen Boianischen GesellscJiaft, 1S97, vol. 15, t. 25, 

 h> 1 



Stauroneis phoenicenteron. When examined at a magni- 

 fication of a few hundred diameters, the valve of this diatom is 

 seen to be prettily marked with black dots ; but when magnified 

 2,000 to 3,000 diameters and very accurately focused, the black 

 dots are seen to be optical effects produced by the membrane 

 closing the meshes of the grating. This membrane is slightly 

 sunk below the general level of the surface of the grating so as 

 to form shallow pits. When viewed with the light quite central, 

 without a stop, the bars of the grating appear very much stouter 

 and the meshes smaller and not so well defined as they do by 

 other methods of illumination, and I have quite failed to detect 

 any trace of pores in the membrane by this method. But when 

 oblique illumination is used, either by means of Powell & Lea- 

 land's chromatic immersion condenser or by a Leitz dark-ground 

 illuminator, in such a manner that it is reflected from the under 

 surface of the cover-glass upon the diatom, then a pore in the 

 centre of the membrane of each mesh or pit is distinctly per- 

 ceptible, and the structure has the appearance represented at 

 fig. 14, which is drawn by means of a camera-lucida from a 

 portion of the grating adjoining the " stauros," at a magnification 



