MICROPLANKTON 211 



The five classes out of the eleven considered by Professor Fritsch that were not represented in our 

 material have been omitted. They include the large thallose, attached marine forms, and others with 

 few, if any marine representatives. The sequence of his scheme is indicated by the roman numerals. 

 To each group a summary of the disposition of individual categories (whether species, genera or 

 larger units) that we recorded, has been added : 



I Chlorophyceae. With two species of Trochischia, rarely and in small numbers only. 



II Xanthophyceae. Halosphaera only, extremely local. 



III Chrysophyceae. With the Coccosphaeriales lumped here because they cannot be adequately 

 sampled by nets (numerically they are a most important group in warmer seas, as nannoplankton 

 methods have shown). Phaeocystis, here mercifully less abundant than in colder seas. 



Silicoflagellata. With two species, never very abundant but widely distributed. 



IV Bacillariophyceae. With ninety-five species or generic categories. 



VI Dinophyceae. With forty-two categories, although only the Ceratia and a few most obviously 

 important of the other thecate forms were identified down to species. 



XI Cyanophyceae. Only one important species here: Trichodesmium thiebautii. 



From the 'Meteor' results (Hentschel, 1936) we know that the Coccosphaeriales, smaller Dino- 

 phyceae and even some of the Chlorophyceae, would figure much more prominently in relation to 

 members of the other groups, if we were considering 'ideal' samples in which nanno- and micro- 

 plankton forms were represented with equal fairness. On the other hand, Halosphaera and the silico- 

 flagellates belong to groups wherein few marine species are known, whatever method of collection is 

 adopted. These limitations, especially those implicit in the choice of the net method, our reasons for 

 which have already been mentioned, led to the decision to treat the phytoplankton under three main 

 headings: Diatoms (Bacillariophyceae), Dinophyceae and 'other Protophyta' or 'other plants'; that 

 is, in descending order of their abundance in these samples, in our main list and primary tables and 

 diagrams. 



Within these groups the order of genera adheres to recent systematic practice, though the more 

 detailed description of the diatom flora has involved some arbitrary arrays to be described later. 



In the main list the genera of diatoms follow the classification proposed by Hendey (1937), but the 

 species within each genus are arranged alphabetically, because the subgeneric arrangements proposed 

 for some large and important genera are still highly debatable. The Dinophyceae follow the sequence 

 used by Schiller (1933), and the heterogeneous assemblage of 'other plants' the order in which their 

 main groups occur in Fritsch's general classification of the algae. 



Chief among the many works consulted for identifications were the Nordisches Plankton series and, 

 for the diatoms, Lebour (1930), Hustedt (1927-37), Hendey (1937) and Boden (1950). For the Dino- 

 phyceae, Lebour (1925) and Schiller (1933) were of the greatest assistance. 



The Protozoa and Metazoa have been treated less thoroughly than the phytoplankton in these 

 samples because the larger ones were better sampled by coarser nets. The genera of Tintinnoinea were 

 determined with the aid of Kofoid and Campbell's Monograph (1929), but to have done the same for 

 the Radiolaria (equally important at a few stations) was beyond my capacity (T.J.H). Similarly, 

 among the Metazoa, while some individual species of local importance were recorded separately, the 

 Nauplii and other larval categories are quite unequal in systematic status. These diverse groups of 

 animals are listed as nearly to the generally accepted taxonomic order as their nature permits. 



The diatoms being by far the most abundant organisms in these samples, are discussed in greater 

 detail than the other main groups, and hence we endeavour to make plain the inter-relations between 

 systematic classification, and arbitrary arrays used merely as an aid to presentation of data, at the 

 outset. 



