163 



On Diatom Structure. 



By T. F. Smith. 



(Bead July 22nd, 1887.) 



Plate XV. 



It will be within the recollection of the members of this Club 

 that when I exhibited here last month what was presumed by me 

 to be Coscinodiscus centralis I was met, on the authority of Mr. 

 Morland, with the objection that it was not Coscinodiscus centralis 

 at all, but concinnus or, I think, angulatus. I did not at the time 

 profess to have knowledge enough of the species of diatoms to 

 decide that question, and even if I had done so the principal point 

 would still remain doubtful, whether the diatom fio-ured bv Messrs. 

 Nelson and Karop and that exhibited by myself were identical. 



Since then Mr. Morland has given me a slide, mounted by him- 

 self, which he considered was Coscinodiscus centralis, and Mr. 

 Sturt has lent me the slide from which Mr. Karop's drawing was 

 made, and I am now in a position to state that, whatever may be 

 the names of the two specimens, they certainly are not the same, 

 and the structure is altogether different. Having stated this fact, 

 my surmises about their having mistaken one structure for another 

 and focussing through the wrong side of the diatom, falls to the 

 ground, and I now beg to express my regret to these gentlemen for 

 challenging their reading of the structure before making quite sure 

 that we were not speaking about different diatoms. 



I am now, however, in a position to speak with certainty about 

 the specimens I show to-night and theirs being identical ; and, 

 having examined them carefully, I am still of opinion that the 

 perforated membrane in Messrs. Nelson and Karop's drawing is 

 shown on the wrong side of the hexagons. There is no question 

 this time of focussing from the wrong side of the diatom, but, I 

 think, of taking too deep a focus and of failing to take note of what 

 was outside before the image disappeared. I find it very easy to 

 get the appearance shown in the figure, and it would be easy to sit 

 down half a dozen times and rise up again from the microscope 



Journ. Q. M. C, Series II., No. 20. 13 



