164 T. F. SMITH ON DIATOM STRUCTURE. 



without suspecting it not to be the true structure. It is no reflec- 

 tion, therefore, on these gentlemen's skill that they have produced 

 this appearance ; but there is an evolution of a knowledge of 

 structure as well as an evolution of structure, and it has taken me 

 a matter of nine months to arrive at my present ideas of the 

 structure of the diatom in question. This diatom was perfectly 

 familiar to me in appearance, although not in name, and I have 

 spent many hours in trying to spell out the meaning of the ap- 

 pearance it presents, but it is only lately, after a study of fractured 

 specimens, that I am able to remain satisfied with my reading. 

 Having convinced myself.it now remains for me to try to convince 

 others, and for that purpose I have two specimens of Coscinodiscus 

 centralis here to-night, one being from the slide given me by Mr. 

 Morland and the other from a spread slide of the Nottingham 

 Deposit, mounted and lent me by Mr. Cole. 



I have also endeavoured to draw the structure that you may know 

 what to look for, and decide for yourselves whether it is correct when 

 identifying it under the microscopes. In PI. XIV., figure 1 shows 

 the centre of the disc, and is supposed to be seen with the outer 

 membrane removed and looking down on the inner layer of eye- 

 spots. Figure 2 shows the outer perforated membrane, the relation 

 of the different parts of which 1 will try to point out on the black- 

 board. In doing this I am obliged to make a more symmetrical 

 arrangement than what is seen in the diatom, to conceal my want 

 of skill in drawing. The hexagons in the specimens seem to be 

 thrown together anyhow, but when I also drew them anyhow and 

 tried to put on the superstructure the result was deplorable. You 

 draw first your hexagons and then place a round dot at each 

 corner large enough to conceal the junction. These dots represent 

 the imperforated part of the outer membrane, and form a ring of 

 six on the top of each hexagon. Now draw another circle in the 

 centre of each ring of dots, bring down a star-like point from the 

 circumference of each dot to the outer edge of the inner ring, and 

 the membrane is complete. There are indications of the centre 

 circle having two or three perforations like those shown by Messrs. 

 Nelson and Karop, but I have not put them in my drawing as it 

 would tend to confuse the structure. Figure 3 in my drawing 

 shows a part of the disc with four or five of the hexagons torn out 

 of the centre, and Figure 4 the same part with some of the outer 

 membrane remaining projecting over the space from which the 



