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On Diatom Structure. 



By Henry Morland. 



(Bead May 28th, 1886.J 



It will be remembered that at the ordinary meeting of this Club, 

 held on February 26th last, Mr. Nelson exhibited a model, show- 

 ing the structure of the valve of Navicula Durrandii, Kitton, and 

 also added a few remarks on the structure of diatom valves in 

 general. Although the members of this Club were so invited, 

 there was no discussion, and the matter dropped. In invit- 

 ing discussion, our President mentioned my name, but at the 

 moment I did not feel equal to the occasion, more especially as I 

 had never paid any particular attention to the structure of N. 

 Durrandii, besides which, as far as my judgment went, I was 

 perfectly in accord with Mr. Nelson's theory, viz., that the " dots " 

 seen on this particular diatom were nothing more nor less than so 

 many minute perforations. I have since examined the diatom 

 more carefully, and if Mr. Nelson's model exhibited the channels, 

 from perforation to perforation on the inner surface of the valve, 

 as running crossways to the length, then he and I are as one so 

 far as relates to the model itself ; but I cannot agree with his 

 views regarding the median line, or " raphe," which, so far as I 

 understood him to say, he looks upon as merely a thickening for 

 strengthening the valve generally. With regard to this point, I 

 may state that certain diatomists consider that true " raphes " are 

 simply clefts with thickened borders, and in the " texte " of Dr. 

 Van Heurck's " Synopsis of the Belgian Diatoms," page 37, 

 there is a figure of a section of Navicula Dactylus, prepared by 

 M. W. Prinz, which shows this cleft very distinctly. I must not, 

 however, confine myself to quoting authorities, but, if possible, 

 give my reasons for acknowledging such authorities. In the pre- 

 sent instance, I have sometimes noticed, when examining this 

 " raphe," that it has two borders, in consequence of the cleft being 

 slightly oblique, one of which, under a high power, will be seen to 

 be on the " upper " surface, whilst the other is on the " inner," 



Journ. Q. M. C, Series II., No. 16. a a 



