16 



unknown, and we, therefore, dare not attempt to decide whether it 

 is of Neritic or Oceanic origin. It belongs to cokl seas, but, during 

 the winter, may be found down in the North sea, and even in small 

 numbers, in the Christiania Fjord. 



A glance at our Tables will shew that these Neritic, Arctic 

 forms are but met with, here and there, in small numbers, otherwise 

 all the samples contain but a non prolific Oceanic Plankton (Cos- 

 ,cinodiseus-species). 



On the whole it will be seen that tbe quantity of Neritic 

 Plankton decreases in richness from the Lim Fjord to the Cattegat 

 and Skagerak; is still poorer on our North Sea littoral, and off 

 the coast of Nordland dwindles down to a minimum. It cannot be 

 due to the change to high northern latitudes. As we have stated, 

 a very rich flora of Neritic Diatoms is to be met with in Karajak 

 Fjord in North West Greenland. It can, therefore, but be due to 

 the bottom conditions, and agrees with our views, that the Neritic 

 Diatoms demand such conditions of life, that their spores are able 

 to sink to the bottom, be preserved there, and subsequently 

 aroused again for development. 



The Oceanic Organisms form a contrast to the Neritic. 



The conditions necessary to existence in the open seas are 

 less varying than in by the land. The temperature may rise and 

 fall with the seasons, or as the bodies of water advance; but the 

 variations proceed slowly. Still more slowly do the changes in the 

 salinity of the ocean occur. 



The ocean, therefore, also possesses a Plankton much more 

 uniform than that met with on the coasts. So long as it remains 

 in the same current, the composition of the Plankton varies very 

 slightly. 



The Pelagic organisms do not require any means of protection 

 against the variations in the temperature and salinity. Each indi- 

 vidual species may be adopted for life within very narrow limits 

 of temperature, yet nevertheless find an immense area in which it 

 can develop itself abundantly. 



The Pelagic Diatoms which we shall specially treat of, may, 

 like other diatoms, become developed in enormous numbers at 

 certain times. When a rich Diatomic Plankton is met with in the 

 ocean, it will be found to contain, as a rule, one or some few 

 species in overwhelming numbers. 



According to SchIitt [93]. there live, dtiring summer, in the 

 Irminger Sea, vast numbers of Thalassiotkrix longissima, and often, 

 in the North Atlantic, Choetoceros decipiens or Ch. horealo may be 

 met with developed in equally great numbers. Other species may 

 occur amongst these masses, but often in quite an insignificant 

 number. 



At other times great stretches of ocean can be almost entirely 

 free of Diatoms. In such a case the conditions for the development 

 for these organisms can hardly be favourable, as some individuals 

 are to be met with everywhere which, if the conditions were favourable, 

 could be the origin of a rich vegetation. 



The Oceanic Diatoms which are of some importance in the 

 Norwegian Northern Ocean are not many. 



As we will set aside rare species, we need but to take the 

 following 16 species into account: 



Coseinodiscus concinwas, W. Sm. 

 „ oculus iridis, Ehe. 



„ . radiatus, Ehh. 



stellatus, Rop. 

 Bhizosolenia styliformis, Bbightw. 

 ,, semispina, Hensen. 



;, Shruhsoli, Cl. 



,, alata, Brightw., /. gracillima 



„ ohtusa, Hensen. 



Chwtoceros horeale, Bail. 



(including Gh. BrigMwellii, Cl.) 

 „ criophilum, Caste. 



(including Ch. convolutum, Castb. = Cli. Brighhvellii, 

 Gean, which is however regarded, by Gean, as a 

 different species). 

 „ cur r ens, Cl. 



„ atlantioum, Cl. 



„ decipiens, Cl. 

 Thalassiotkrix Frauenfeldii, Geun. 



„ longissima, Cl. & Geun, 



It would not appear as if all of these species were favoured 

 by the same outward conditions. As a rule not more than one or 

 some few species are to be met with in quantities, whilst the others 

 then are but secondary elements of the Plankton. 



It is this circumstance, whicli gave Cleve the idea of separa- 

 ting his types of Oceanic Plankton (see inter alia [97], p. 4), which 

 we in the following enter into at greater lengtli. Tliere are espe- 

 cially five species which can appear in such numbers as to give 

 the Plankton its character; namely 



Coseinodiscus oculus iridis 



Thalassiotkrix longissima 



Chwtoceros decipiens 



Rhizosolenia styliformis 



B'hizosolenia alata (f. gracillima). 



Those hydrographical conditions which in different instances 

 gives one of these species the priority of the others, have not yet 

 been investigated. In many cases the temperature appears lo play 

 an important part, even if this is not per se capable of explaining 

 everything. 



The Coseinodiscus species, thus occur on oui- shores, especially 

 during the coldest period of wintci ; after these come Thalassiothrix, 

 and somewhat later on the Oceanic Chcetoeeros species, whilst Bh/i- 

 zosolenia styliformis, arrives with the Atlantic current during the 

 summer and autumn, and Bh. alata. thrives best in the very warm 

 surface layers which the Baltic current conveys along the coast 

 (see Table 1) during summer. 



The results derived from the two "Heimdal" expeditions in 

 May 1896 and 1897, also agree with the above. 



The distribution of the Plankton is graphically described on 

 PI. 4, in the Hydrographic Profiles. 



Under each Hydrographic Station will be found two columns, 

 of which the first, on the list, denotes the surface Plankton, the 

 other, the contents of a sample taken in a net drawn perpendicu- 

 larly to the surface from a depth of 100 metres. The Oceanic 



