201 



On the Formation op Diatom Structure. 



By E. M. Nelson. 



{Read January 21th, 1888.) 



Plate XVIII. 



I have some misgivings in bringing this matter a second time to 

 yonr notice, neither would I do so had I not lately found some 

 fresh examples which throw additional light on the subject. 

 Further, I believe we are on the verge of a new departure in 

 the field of microscopical work, viz., illustration by means of 

 lantern pictures from photo-micrographic positives. I have brought 

 a lantern with me to-night, and have chosen this as my first sub- 

 ject, illustrated by photo-micrographic positives of my own manu- 

 facture. 



My first picture is that of a portion of Coscinodiscus Janischii 

 (Plate XVIII., Fig. 1), a drawing of this I have already exhibited 

 here. It shows an increase of structure from the centre to the 

 margin, an areolation is at first indented by the formation of a 

 process, in the next areolation, counting towards the periphery 

 the process is longer, in the next it has nearly reached the 

 opposite side of the areolation, in the next row you will observe 

 that the single areolation has become two ; these increase in size 

 and divide again. This to my mind indicates a growth from the 

 centre outwards. 



Now we come to a series of six photographs of fragments in the 

 Briinn deposit. 



The first, Fig. 2, you will probably recognize as the one with the 

 fiddle / mark described in the paper " On the Finer Structure 

 of Certain Diatoms," by Mr. Karop and myself.* Before passing 

 on let me ask you to notice the accuracy of Mr. Karop's drawing, 

 the identical areolation being easily recognizable. 



In Fig. 3 the areolation becomes more circular, the central cut 

 in the/ has joined the openings at the ends which causes it to 



* " Journ. Q. M. C," Ser. ii., Vol. iii., No. 18, PI. IV., Fig. 7. 



