T. F. SMITH ON DIATOM STRUCTURE. 165 



hexagons are wanting. I am sorry to say this last drawing is as 

 bad as can be, and fails altogether to convey the appearance of the 

 torn structure under the microscope. That part in the microscope 

 can be readily seen from its being paler, proving that the dark 

 colour of the ring of spots is due to the hexagons ordinarily under- 

 neath. This is all I have to say about this diatom to-night, but 

 there are other appearances that I have lately seen that I have not 

 embodied in this paper because they are new to me, and I am not 

 sure how far they are true. I shall be pleased, however, if the 

 members will look at it for me and give me their opinions. 



There now remain two little matters connected with my last 

 paper that I should like to mention. The first is my opinion that 

 the figure of Aulacodiscus cellulosus given in the Journal by 

 Messrs. Grove and Sturt confirmed my views of the structure of 

 Aulacodiscus Kittonii, although I had not examined it. Since 

 then Mr. Sturt has kindly lent me the slide, and I find the 

 structure is exactly what I said it would be, viz., an inner layer of 

 eye-spots, with hexagons stuck on the top, and an outer perforated 

 membrane on the top of the hexagons. 



Secondly, I exhibited a Coscinodiscus with an outer perforated 

 membrane, stuck on the top of the spines which project one 

 from each corner of the hexagons. The specimen was a fractured 

 one, and did not show the membrane all over, but my reason for 

 choosing it was that it showed the broken edge of the membrane 

 projecting over the hexagons. One gentleman that night could 

 not conceive a membrane being in that position, and thought it was 

 part of another diatom slipped on to the one I was showing. I 

 tried to convince him that it was not so, but don't think I suc- 

 ceeded. Since then I have had a conversation with Mr. Morland 

 on the subject, and find that he also is sceptical and puts down the 

 appearance to a granulation of the medium in which the diatoms 

 are mounted. I may state that some time ago I had a bit of a brush 

 with Mr. Morland in the " English Mechanic ' : on the existence 

 of a structureless membrane on the inner side of Arachnoidiscus, 

 and must admit that in that encounter I came off second best, and 

 this, I suppose, is what makes him think I am again in error. Both 

 these objections are perfectly legitimate ones, and having advanced 

 my opinions it is my duty to prove my case. To do this I have 

 brought the same slide to-night, and am fortunate enough to have 

 here specimens not far from each other, one showing the broken 



