348 



DEPTHS OF THE OCEAN 



species themselves. Our investigations at different seasons, 

 both in coastal waters and in the North Atlantic, have shown 

 us that the flora of each locality is constantly changing. One 

 species succeeds another as month follows month, and different 

 societies predominate in the same locality at different seasons. 



Along the west coast of Norway, for instance, we find a 

 flora during the winter, from December to February, scanty in 

 numbers, but consisting of many species, and mainly composed 

 of true Atlantic forms (Styli-plankton), which reach their northern- 

 most limits in the dark months of the year. About March or 

 April the temperature attains its minimum, and great quantities 



Fig. 245. 

 a, PlanktoTiiella sol, and b, Gossleriella tropica, from the Atlantic. (Schiitt. ) 



of diatoms are then produced, which are mainly arctic. Some- 

 times these are almost entirely neritic, and sometimes there is a 

 considerable addition of oceanic species. As often as not it 

 is the species of Thalassiosira and Coscinodiscus which first 

 appear, and then comes Chcetoceras, C. debile being usually the 

 form found on the west coast, C. constiHctum preferring the 

 Skagerrack. In May the predominant form is generally 

 Leptocylindrus daniciis. We next get a period in June when 

 the prevailing forms are oceanic, Ceratimn longipes at that time 

 attaining its maximum development and characterising the 

 flora. In August the warmth-loving peridineae begin to be 

 more and more numerous, Ceratuim fusus, C. furca, and 

 C. tripos being then much in evidence, and continuing to increase 

 until October. Finally, in November we get a comparatively 



