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DEPARTMENT OF THE NAVAL SERVICE 



persist in considerable abundance over the whole area throughtout the year ; secondly, 

 those which occur occasionally at all seasons; and thirdly, those which attain a 

 marked predominance at one season and then either entirely disappear or occur at 

 rare intervals. 



Bathymetric Eange. 



Station 6. A comparison between surface and 5 metre tows. To ascertain the 

 more favourable depth for the gathering of material a comparison was made between 

 the numbers obtained in monthly tows at the surface and at a depth of 5 metres. Each 

 species was considered separately and each presented the same irregularity of distribu- 

 tion. Most frequently, however, the greater numbers came from the lower level, for out 

 of 183 comparisons made, the five metre collections proved the greater in 103 cases. No 

 species showed a preference for the surface water, nor did any fail to appear in them. 

 A synopsis of the results for eleven of the most abundant genera will be found in 

 Table Vni. 



TABLE VIII. 

 Comparison of Surface and 5m. Tows. Station 6. 



Stations S and 6. — Station 3 offers the best conditions for a study of the batliy- 

 metric range, since at that point the water has a depth of 175 metres; at Station 6 it 

 ranges from 26 to 30 metres. At the former station eleven samples were taken on 

 July 31, at intervals of 10 or 25 metres. Later an estimate was made in the following 

 manner of the average diatom content of 50 c.c. at each level. From each of the 

 eleven samples, four volumes of 50 c.c. each were centrifuged for half an hour, it having 

 been previously ascertained that that period sufficed for the extraction of all the 

 plankton organims. The water was then siphoned off leaving the residue in 2 c.c. The 

 organisms were again counted in the Rafter cell, but in this case the cover slip was 

 divided into forty squares, each measuring 25 sq. mm. The frustules of each species 

 were counted in 10 squares in each of the two slides made from a preparation ; and the 

 average of these multiplied by 80 gives the total content of tlie 50 c.c. The average 

 results obtained from the four similar 50 c.c. samples drawn from each will be found 

 in Table IX. 



