480 Trygve Braarud 



During the last few years observations have been made on approximately 50 species 

 (for literature see Halldal and Markali, 1955 b). A new terminology for the descrip- 

 tion of the coccolith morphology has been introduced, and the first outline has been 

 drawn up for a coccolith typology based upon electron microscope observations 

 (Braarud, Deflandre, Halldal and Kamptner, 1954; Halldal and Markali, 

 1955 b). To a large extent the old coccolith types have had to be discarded. It has 

 also been revealed that some of the old species actually include more than one taxo- 

 nomical unit. It seems obvious that, in future, electron microscopy will be indispens- 

 able when describing new species of this group. 



Electron microscope observations have disclosed an unforeseen variety in the 

 microstructure of the coccoliths, and definite taxonomic groups based upon clear-cut 

 differences in the microstructure of the coccoHths can already be distinguished. In 

 PI. I and II are shown examples of some coccolith types which illustrate the striking 

 diversity in their architecture. Species which, because of their coccolith micro- 

 structure, must be assumed to be closely related, may exhibit increasing differentiation 

 in the coarser features of their coccolith morphology. As an example PI. II, 1-6, 

 shows coccoliths from four species, all of which have coccoliths of the holococcolith 

 type, ranging from Crystallolithus hyalinus with its very simple coccoliths to Homo- 

 zygosphaera tholifera with its elaborate " mitras ". In this case the microstructure 

 of the coccoliths, which can be revealed only through electron microscopy, must 

 form the basis for a revised systematical treatment of the group. The extremely 

 different microstructure of such coccoliths as shown in PI. I and II may, in time, give 

 proof of a more complex origin of the coccolithophorids than hitherto suspected. 



The study of this group with the electron microscope is at its beginning. Much 

 work is still ahead before full advantage can be taken of this new tool, but already 

 one may point to useful applications of these results in other fields, as for instance 

 the study of coccoliths in rocks and in oceanic sediments of different ages. 



Coccoliths form a part of sediments in the oceans, and of calcareous rocks from 

 various periods (Deflandre, 1952). In most cases the coccoliths are found detached, 

 but since observations on the planktonic forms of today have established the specific 

 nature of the coccolith structures, their identification is now possible through electron 

 microscopy. Considerable work has already been done on fossil forms (for literature 

 see Deflandre and Pert, 1954). In some cases their coccoliths have been found to 

 be identical with those of species living in the oceans today, while in other cases 

 they are different from any yet observed in living plankton. 



In examining stratification in oceanic sediments the study of the small coccoliths, 

 which hitherto have been unidentifiable, may become most useful. 



The dinofiagellates, another important group of marine phytoplankton, for the 

 most part are large enough to be studied adequately in the light microscope. How- 

 ever, some oceanic species are so small that essential morphological features escape 

 observation. Exuviaella baltica, a species which has a wide distribution in the North 

 Atlantic and which forms an important component of the summer populations 

 (Braarud, Gaarder and Grontved, 1953; Halldal, 1953), has been shown to have 

 a spiny surface structure which was quite unexpected from observations in the fight 

 microscope (see PI. II, 7). This morphological detail may be of considerable ecological 

 importance in view of the fact that the spines increase the absorbing surface of the 

 cell. In part, an explanation of the wide distribution of this species and its growth in 



