. Fig- 3- ANALYTICAL KEY TO THE GENERA The construction of a key is a matter of great difficulty in diatoms, because of the difficulty of finding adequate expression for the definitive characters, to make them com- prehensible to the uninitiated, and because the size of the organisms does not allow their various aspects to be examined with ease. The following key is mainly for the field worker, based upon characters observed in fresh material or in material carefully pre- served in formalin. Great care must be taken in consulting section 1 of the key. In section 1 are included all the main div
. Fig- 3- ANALYTICAL KEY TO THE GENERA The construction of a key is a matter of great difficulty in diatoms, because of the difficulty of finding adequate expression for the definitive characters, to make them com- prehensible to the uninitiated, and because the size of the organisms does not allow their various aspects to be examined with ease. The following key is mainly for the field worker, based upon characters observed in fresh material or in material carefully pre- served in formalin. Great care must be taken in consulting section 1 of the key. In section 1 are included all the main divisions of the diatoms observed in the Antarctic. Section 1 is necessarily large, as the old division of diatoms into two groups, viz. Centricae and Pennatae, which is wholly inadequate in the case of plankton diatoms, has been abandoned, and the first section has been framed upon altogether different lines to include the genera of zygo- morphic diatoms which form such a large part of the phytoplankton. Correct interpre- tation of the characters set out in section 1 will take the worker directly to the group of
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