308 



On the Formation of Diatom Structure, II. 



By E. M. Nelson. 



(Head September 28th, 1888.^1 



Plate XXIV. 



In mj- previous paper on this subject, which you thought 

 worth}' of insertion in the Journal,* I expressed an opinion that 

 the structure of Coscinodiscus Janischiif indicated a growth 

 from the centre outwards. I have lately found further evidence 

 in confirmation of this opinion. 



When examining a Coscinodiscus diorama, with my apochro- 

 matic J, I found the following details : — 



The immediate centre of the valve is composed of structure- 

 less silex. 



Proceeding from the centre to the periphery we come to the 

 primary areolations. 



They are small at first and well separated from each other, 

 they get larger and closer the further you get from the centre, 

 and eventually they assume the usual polygonal form. 



The secondary structure is exterior to the primary areolations 

 which are nearest to the centre of the valve, as in Plate xxiv, 

 Fig. 1. 



As we proceed outwards we find the secondary structure on 

 the edge of the primary areolations, as in Fig. 2. 



At the periphery the secondary structure is inside the primary 

 areolations, it resembles Fig. 3, which is like a fine Coscinodiscus 

 aster omphalus.% It will be noticed that in Fig. 3 the structure 

 is complete with the sieve like membrane inside the circle of 

 small holes. I lose this sieve membrane a short distance from 

 the periphery towards the centre of the valve ; none is shown 

 at Fig. 2. 



There is yet another point, viz., no eye spots can be detected 



* " Jour. Q.M.C.," Nor. ii, Vol. 3, p. 201. 



f Plate xviii. Fig 1, 



X " Jour. ({.M.C.;' Ser. ii, Vol. 2, 1'late xvii, Fig 1. 



