126 T. F. SMITH ON DIATOM STRUCTURE. 



Here I should like to stop, but two other papers, with their 

 plates, have been brought before you by Messrs. Nelson and 

 Karop, which to me, in some parts, seem to confuse and 

 unsettle our ideas of structure already fixed, in a certain way, 

 by the consensus of microscopic opinion. 



These drawings of the finer structure of certain diatoms, 

 admirable as they are as illustrations, give, I am afraid, in 

 three instances at least, misreadings of the structure, and I 

 hope these gentlemen will not think me presumptuous if I 

 venture to point out where I think they are wrong. 



In Fig. 1 of their plate, given in the May number of the 

 Journal of this Club for last year, is a drawing of the finer 

 structure of Coscinodiscus aster omphalos, and were the figure 

 only given, there would be nothing more to say, showing 

 as it does correctly, the finer structure of that diatom, but the 

 letterpress goes on to say : " This diatom, although consisting 

 of a single siliceous membrane, has a double structure, viz., 

 coarse and fine areolations, the latter within the former." By 

 this I understand they mean that both structures are on, or 

 nearly on, the same plane ; but, as a matter of fact, each single 

 disc of this diatom has three thicknesses of structure, each 

 differing from the other. You have first the outer membrane, 

 as figured ; next, underneath this, a layer of hexagonal cells ; 

 and then an inner plate of so-called eye-spots ; and I can only 

 account for the outer membrane only being seen by assuming 

 that the j^" oil-immersion used by these gentlemen had not 

 working distance enough to allow them to focus through the 

 whole thickness of the disc. On stand No. 1 you will find a 

 disc of Coscinodiscus asteromphalos with bits chipped out of the 

 middle, and you will be able to see for yourselves the different 

 layers of structure. In the part shown the under membrane 

 with the hexagons, is intact, but in another part the whole of 

 the layers are torn through. You will also observe that the 

 structureless part is the part torn away, proving that the space 

 between the areolations is the thinnest part of the membrane. 



In Fig. 1 of Plate IV. in the last number of the Journal, is a 

 drawing purporting to give the finer structure of Coscinodiscus 

 centralis, and here certainly are hexagons, but on the wrong 

 side of the outer membrane. My mind was greatly exercised 

 over this figure, as 1 had lately been looking at a great number 



