INTRODUCTION 



159 



Hydrological data obtained during April-May 193 1 on the 

 20th W meridian 



METHOD OF WORKING 



Before a critical examination of the species could be made it was necessary to remove 

 the larger crustaceans, fragments of animal and vegetable detritus, and to wash the 

 diatoms free from salt. This was accomplished by passing the sample through a small 

 No. 20 wire sieve, which removed the larger fragments, and then conveying the filtrate 

 to a washer, illustrated in Fig. 2. 



This apparatus, which was especially designed for the purpose, and has given 

 excellent results, consisted of a glass cylinder A, \\ in. in diameter and 4 in. in length, 

 fitted with a rubber cork through which passed two glass tubes of ^ in. bore, one, B, 

 passing just inside the cork and bent in the manner shown, the other, C, furnished with 

 a pipette end and inserted through a rubber cork into a small glass tube D of f in. bore, 

 which had its lower end flanged. The flanged end was covered with a piece of bolting silk 

 (200 mesh) and securely tied. 



The filtrate was allowed to stand until the diatoms settled ; the clear portion was then 

 decanted, and the remainder agitated and gently poured into the small glass tube D 



