Journal 



OF THE 



NEW-YORK MICROSCOPICAL SOCIETY, 



Vol. VII. APRIL, 1891. No. 2. 



ON THE STRUCTURE OF THE PLEUROSIGMA VALVE. 



BY T. F. SMITH, F.R.M.S. 

 {Read January id, 1891.) 



On September the 28th, 1888, I had the honor of reading be- 

 fore the Quekett Club a paper bearing the above title, in which I 

 attempted to prove that the supposed single plate of silex form- 

 ing the pieurosigma valve was, in reality, built up of two or three 

 layers of structure. 



That this idea of mine was not a new one I subsequently dis- 

 covered and admitted in a paper on the same subject, read by me 

 before the Royal Microscopical Society, in which I quoted the 

 remarks of Herr Grunow and Mr. Kitton on the subject. I also 

 have taken the opportunity in this paper of bearing testimony to 

 the labors of Dr. Jacob D. Cox in working out the structure of 

 the diatom-shell ; who, although he makes no positive statement 

 on the pieurosigma valve, says that certain appearances are " not 

 inconsistent with a double structure." 



Incidentally I may say that after arriving at a certain stage of 

 my work I searched through the collections of photographs of 

 diatom- structure belonging to the Royal Microscopical Society, 

 and found Dr. Cox's prints of P . angulatum to be the only ones 

 giving a correct rendering of one of the layers — the one shown in 

 my slides Nos. 35, 2),^^ and 37 of that diatom. 



The reason is very simple: This layer can only be distinguished 

 from the one on the other side of the valve by the use of a large 

 cone of light; and having so distinguished it, I think America can 



