302 T. F. SMITH ON THE VALVE OF PLEUROSIGMA. 



self, who has done so much work on the diatom valve, has never 

 seen the " postage stamp " edge on a fractured specimen of 

 Pleurosigma formosum, but only the usual series of circles 

 bulging outwards. For myself, I may say that although 

 prepared from analogy to regard the structure of Pleurosigma 

 as perforated, the absence of anything like notches at the 

 broken edge of some of them caused me to consider it as an 

 open question only, and to put down the appearance of "beads" 

 in this position as due either to an aggregation of particles out- 

 side the resolving faculty of our lenses, or to a confusion of 

 image arising from the existence of more than one layer of 

 material. 



With me the matter rested there, but the recent acquisition 

 of the highest optical appliances at present attainable, has led 

 me to try to solve the problem afresh, and I beg to-night to lay 

 before you the results of my investigations. 



The valve over which the toughest battles have been fought 

 has been that of Pleurosigma angulatum, but I propose to begin 

 with Pleurosigma formosum, as being the coarser form, and also 

 the one likely to yield up the most interesting results. 



On looking at this diatom, mounted in balsam, a most strik- 

 ing object is seen; but, consequent on the finer structure being 

 obliterated by that method of mounting, for all points of study 

 it is useless. I suppose it is for this reason that but little 

 is read about it now ; for one has only to look at one of the 

 plates in Dr. Carpenter's work, showing the different appear- 

 ances it presents when illuminated from different directions, to 

 see how it must have puzzled microscopists when formerly 

 mounted dry. 



To make out the structure with a dry lens, on a dry-mounted 

 slide, is almost impossible, and even when examined thus with 

 Zeiss' apochromatic oil-immersion, the first view seems confusion 

 worse confounded : each valve differing from its fellow, and 

 agreeing in no particular — not even in colour — with the same 

 diatom mounted in balsam ; a further study, however, reveals 

 that what appeared so simple, when mounted in the latter, is very 

 elaborate in reality, and the apparently simple plate of silex 

 resolves itself into several layers of structure. The two outer 

 layers — one on each side of the diatom — differ in their ultimate 

 structure, bat primarily are composed of a square grating with 



