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



to others the} 7 evidently seem larger, as one friend said he thought 

 that about five of them just touching one another would extend 

 right across one of the meshes of P. balticum, but even at that 

 rate they would not be more than 1/1 80,000th of an inch in dia- 

 meter, whilst I think they cannot be more than l/200,000th 

 of an inch in size. 



With regard to Mr. Smith's statement that there is a second 

 grating, I have not the slightest doubt that the transverse bars 

 form such a grating, but I have not seen it separately from the 

 outer grating. In Knowledge for 1911, p. 334, Mr. Smith 

 reproduces a photograph of P. balticum in which the longi- 

 tudinal strengthening bars are shown and are there called 

 " fibrils," a term which Mr. O'Donohoe has also adopted, but 

 which to me seems wholly inapplicable, as they appear to me to be 

 supporting structures for the delicate membrane and in no sense 

 ultimate structures. It may not be out of place here to point 

 out that the membrane I speak of and illustrate is a totally 

 different thing from that which Mr. Smith in the Joum. Q. M. 0. f 

 Ser. 2, vol. 3, p. 301, t. 3, fig. 5, and in Knowledge (1911), pp. 289- 

 93, and 221-35, and (1912) p. 371 describes and figures as a 

 "delicate membrane' 3 and "torn structure." For it is a 

 matter of great surprise to me that Mr. Smith did not recognise 

 that this supposed "delicate membrane " and "torn structure" 

 has no morphological connection with the diatom. I had sup- 

 posed, previous to reading his paper, that every one regarded 

 this appearance merely as an incrustation cementing the diatom 

 to the cover-glass ; it is of very common occurrence upon 

 Pleurosigma and some other diatoms. I have always regarded 

 it as due to the exudation of a residual salt, which, after 

 boiling in acid, has not been thoroughly washed out of the diatom 

 (and it is indeed very difficult to wash out completely), so that 

 when mounting them on a cover-glass the water outside the 

 diatom evaporates first and the salt then gradually percolates out 

 through the pores of the diatom, and, in drying, fixes it to the 

 cover-glass, and being of low refractive index produces the 

 appearance we so often see. 



It will be noted that there is a discrepancy between my 

 drawings and Mr. O'Donohoe's photographs in the size of the 

 black dots, for although mine are represented at a greater 

 magnification, they are smaJler than in the photograph. This is. 



