PELAGIC PLANT LIFE 347 



species and varieties of the genera Peridinium and Gonyaulax 

 might be employed with equal advantage, but they are more 

 difficult to determine, and so little studied as yet that the 

 determinations of Hensen and Karsten are unserviceable. 

 Owing to so little being known about their distribution, I have 

 decided to ignore them for the present. 



The oceanic species may also be divided into three main 

 groups : — 



(i) Arctic forms, corresponding to Cleve's Tricho-plankton and Arctic oceanic 

 Chaeto-plankton. Most of them occur also in antarctic waters. species. 



Diatoms : Thalassiothrix longisshna, Coscinodiscus subbulliens, CJiceto- 

 ceras criopJnlum, C. boreale, C. convolutum, C. atlanticum, C. decipiens, 

 Rhizosolenia hebetata {seuiispina), Nitzschia seriata. 



Peridineae : Ceratimn arcticum, C. longipes, DinopJiysis gramdata. 



(2) Temperate- Atlantic forms, corresponding to Cleve's Styli-plankton Temperate 

 and Tripos-plankton. The latter of these two designations comprises a oceanic 

 small community of species, which are less exacting as regards salinity, *P^"^^- 

 and are therefore produced in quantities along the coasts of North 

 Europe. 



Diatoms : Rhizosolenia styliformis, R. acuminata, R. alata, Coscino- 

 discus radiatus, C. centralis, C. stellaris, Chcetoceras densum, C. dichata, 

 Corethron criophilum, Dactyliosolen antarcticus, Thalassiosira subtilis, 

 Coscinosira cestrupi, Asteromphalus Jieptactis, Bacteriastrum delicatulu^n, 



B. elongatum. 



Peridineae : Ceratium tripos, C. bucephalum, C. azoricum, C. niacroceros, 



C. interjuedium, C. lamellicorne, C. reticulatum, C. fusus, C. furca, 



C. lineatuni, Dinophysis acuta, D. hastata, D. hommiculus. 



Coccolithophoridae : Coccolithophora pelagica, PontospJicsra Jiuxleyi. 

 Chlorophyceae : HalospJicsra viridis. 



(3) Tropical-Atlantic forms, corresponding to Cleve's Desmo-plankton, Tropical 

 and comprising a series of species, especially peridineae and coccolitho- oceanic 

 phorids. Cleve's guiding form is the blue-green alga Trichodesmium 

 tJiiebaultii. The following are some of the commonest : — 



Diatoms : Coscinodiscus rex, Planktoniella sol, Gossleriella tropica (see 

 Fig. 245), Rhizosolenia castracanei, Chcetoceras coarctatum, Asterolampra 

 marylandica, A. rotula. 



Peridineae : species of Ceratium of all groups {prcelongum, cephalotum, 

 gravidum, cajidelabrum, pennatum, extensuvi, palmatum, massiliense, 

 carriense, and several others), species of Oxytoxuvi and Podolampas, 

 Ceratocoiys horrida, species of Phalacroma, Dinophysis schiittii and 



D. uracantJia, species of Amphisoletiia and Ty-iposolenia, Ornithocercus 

 magnificus, O. quadratus, O. steinii and O. splejididus, Pyrocystis 

 noctiluca and P. fusiformis. 



Coccolithophoridae : Coccolithophora leptopora, species of Syracosphcera, 

 Calciosolenia murrayi, Michaelsarsia elegans, and many others. 



The boundaries of the areas populated by these communities 

 of species are as variable as the limits of distribution for the 



