346 



DEPTHS OF THE OCEAN 



called arctic. Here, too, diatoms predominate, and CJicetoceras takes 



first place. The commonest forms include : — 



{a) Northerly : Ch(£toceras teres, C. constrictuin, C. diadema, C. debile, 



C. crinitum, C. pseudocrinitum, C. scolopendra, C. sociale, C. simile, 



Rhizosolenia setigera, TJialassiosira decipiens, CosciJiosira polychorda, 



Leptocylindriis daniciis. 



ip) Southerly : ChcBtoceras weissflogii, C. contortiim, C. didymuvi, 



C. laciniosmn, C. schnitii, C. curvisetum, C. cinctum, C. afiastoinosans, 



C. radians, Laiideria anmdata, Ceratatdina bergonii, Biddidphia mobi- 



liensis and B. regia, Eucampia zodiacus, Dityluni brightzvellii, Guinardia 



fiaccida, Asterionella japonica, the peridinean Prorocentruvi inicans, and 



the brown flagellate Phceocystis globosa. 

 Tropical (3) Tropical neritic species have had far less study devoted to them ; 



neritic species, g^ii} ^yg j^^y denote by this term a whole series of species that have 



their northernmost limit on the coasts of the Mediterranean. Of these 



we may mention : — 



Chcetoceras furca, C. diversum, C. femur, Hemiaulus hauckii and 



H. heibergii, Detonula scJirdderi, Asterionella notata, Rhizosolenia 



cylindrus. 



The neritic flora off the coasts of the Atlantic in the southern 

 hemisphere has also been comparatively little studied as yet. 

 Still we are justified in saying that the neritic diatoms of the 

 antarctic, from the ice barrier northwards, differ in the main 

 from species belonging to the northern hemisphere. The 

 difference indeed is so great, that hardly a single species is 

 common to both arctic and antarctic waters. The investiga- 

 tions of Cleve, Karsten, and Van Heurck show that the 

 following neritic diatoms may be considered characteristic of 

 the antarctic : — -ChcEtoceras radiculum, Moelleria a^itarctica, 

 Eucampia balatistitun, jFi'agilaria antardica, Thalassiosira 

 anlarclica, and probably several others whose biology is as yet 

 only slightly known. 



Neritic dia- 

 toms in the 

 Antarctic. 



Oceanic 

 species. 



Oceanic plankton algai are much more widely distributed 

 than neritic algae, and it would almost seem from our material 

 that each species may be met with in all the seas of the world, 

 wherever there are favourable conditions of existence. The 

 diatoms are apt to occur irregularly. Sometimes we find 

 enormous quantities of them, and at other times they may 

 be so scarce that it is difficult to detect them. The peri- 

 dinese are more evenly distributed, and this is true especially 

 of the species of Ceraliuni, which are fairly abundant and hardly 

 ever absent from oceanic-samples, unless perhaps in arctic 

 waters. They may well be used as guiding forms to express 

 the character of the plankton. It is possible that the different 



