PELAGIC PLANT LIFE 345 



shallower waters, Halosphara is oceanic, and so also are the 

 species of Trichodesmium ; but there are several blue-green 

 species that are brackish-water forms, and they must of course 

 be accounted neritic [Anabcsna baltica, Nodtilaria spumigena, 

 Aphanizoinenon flos-aqiics). 



Several of the neritic algai practically only occur locally. 

 Detonida cystifera, for instance, appears in the Limfjord in 

 Denmark and along the south coast of Norway, while Litho- 

 desinium juidulatuniy Coscinodiscus granii.Navicula memb^^anacea, 

 and Streptotheca thamensis belong to the English Channel and 

 to the southern portion of the North Sea. I could mention 

 additional examples, but the greater number of them are far 

 more widely distributed. It has been found possible to allocate 

 all the species along the coasts of the Northern Atlantic to 

 three comprehensive main groups, namely, the arctic, temperate, 

 and tropical. This is perhaps rather an arbitrary arrangement, 

 as these groups encroach to a very great extent upon one 

 another ; so that we get northern forms a long way south in 

 the winter, and in the autumn the southern forms extend 

 northwards. Further researches, too, might result in a stricter 

 classification, while it is known that there are species which, 

 biologically speaking, unite the groups, and might with equal 

 reason be assigned to the one or to the other. 



(i) Arctic neritic species are mainly those which Cleve termed Sira- Arctic neritic 

 plankton, and consist principally of diatoms. The characteristic forms species. 

 are the species of Thalassiosira from which this name was derived. 

 They are composed of long strings of short cylindrical cells united by 

 a central thread of slime. Thalassiosira hyalina has its southernmost 

 limit off the north of Norway, while T. gravida and T. nordenskioldii 

 occur in winter as far south as Central Europe. A series of species 

 belonging to the genera Fragilaria, Achnantes, Navicula and Amphiprora 

 are also distinctly arctic forms, and are characterised by having their 

 cells bound together like ribbons. These include Fragilaria oceanica, 

 F. islandica and F. cylindrus, Achnantes tceniata, Navicula septentrionalis, 

 N. vanJwffenii and A", granii, and Amphiprora liyperborea. The 

 usually predominant genus Chcetoceras is only represented by two 

 purely arctic species, namely, Chcetoceras furcellatum and C. mitra. 

 We must likewise add the well-known Biddulphia aurita. Besides 

 these diatoms, there are the peridinean Gonyaulax triacatttha, and the 

 brown flagellate PhcEocystis poucheti, with its naked cells in large slimy 

 round or lobate colonies. 



(2) Temperate neritic species are even more numerous. The warmth- ^J^^I^Pf^'^^^^g 

 loving species fall under Cleve's designation of Didymus-plankton, with "*-» ^c species. 

 CJicBtoceras didymum as the most characteristic form. It is, however, a 

 better arrangement, perhaps, to associate with them a series of other 

 species with a sUghtly more northerly character, that cannot be really 



