SEA-FISHERIES LABORATORY. 103 
the times and conditions of occurrence of these dominant 
factors of the plankton in an average year. 
Our confidence that these samples upon which we are 
reporting in this series of papers are adequate and representa- 
tive, receives support from the fact that the same organisms 
are recorded in much the same quantities year after year, 
and that practically no new forms that really matter turn up. 
Rare species are no doubt of scientific interest, but it cannot 
be too emphatically stated that it 1s the common species that 
are of most importance, those species which by their abun- 
dance in nature play their part in providing fish-food for man 
or in affecting the public health either by keeping the sea clean 
or by causing plagues. 
For the purpose then of arriving at conclusions as to the 
distribution throughout the year of these really significant 
organisms, we may pick out from our records the following 
six species of Copepoda as being undoubtedly the most 
abundant and economically the most important representatives 
of that section of the plankton :—Oitthona similis, Pseudocalanus 
elongatus, Acartia clausi, Temora longicornis, Paracalanus 
parvus and Calanus finmarchicus. In the case of the Diatoms 
it is better to deal with genera, and we may choose the following 
as being the most important representatives of the Diatoms 
in our plankton :—Biddulphia, Coscinodiscus, Chaetoceras, 
Rhizosolenia, Thalassiosira, Guinardia and Lauderia. In some 
of these genera, such as Chaetoceras and Rhizosolenia, there 
are usually several allied species occurring together in any 
large gathering. 
Now, these are the selected Copepoda and Diatoms to 
which we have been paying special attention for the last decade, 
and in regard to which we have given many details in these 
successive Annual Reports.* We shall, therefore, merely add 
* For further details and a summary of the whole matter see Herdman, 
“Spolia Runiana, ITI,” in Journ. Linn. Soc. (1918) ; and for similar informa- 
tion in regard to the English Channel, see Lebour, Jowrn. Mar. Biol. Assoc. 
(1917). 
. H 
