21 



As all these species are really coastal forms, as CLEVE him- 

 self has found out, is it a priori very improbable that not one 

 single species belongs to the shores of the Skagerak. And on 

 closer observation of the individual types this shews itself still 

 more clearly. 



The first Neritic community, the Didymus-PIankton (Nm), occurs 

 in the Skagerak during summer and autumn when the water is warm. 

 In like manner we meet with the Sira-Plankton (Si) when the 

 temperature is at its lowest in the months February to April. 



There are thus two annual maxima in the Neritic Plankton. 

 The one, during the warmest time of the year, formed of species 

 which are most widely diffused on more southern shores (Cluetoceros 

 didymum, Schiittii, Weissflogii, Ditylum Brightwelli, Jiiddulphia 

 mobiliensis &c.); the other, which attains its maximum during the 

 coldest season of the year, formed of northern species, Thalassiosim 

 grainda, Nordenskioldii, Coseinodiscus polychordus, Chcetoceros debile, 

 diadema, sociale, Biddulphia uurita. On the colder, northern shores 

 (Greenland, Spitzbergen), where these species may be met with in 

 great numbers, they attain their maximum during the warmest 

 months of summer. 



It is not alone the Plankton Diatoms that have two such 

 annual periods. A like phenomenon is to be found amongst, for 

 instance, the higher Marine Algae. 



The annuals amongst these are, either the summer forms which 

 develop during the summer, and fructify during the autumn (for 

 instance Nemalion multifidum, Dictyota dichotoma, Cutleria multi- 

 fida, Delesseria ruscifolia, Cystodonium purpurascens, Potysiphonia 

 byssoides), or the winter forms which fructify during the spring 

 (Scytosiphon lomentarius, Dumontia filiformis. and the Monostroma 

 species). Also in respect to these it is certain that the Winter Flora 

 chiefly consists of northern species; the Summer Flora, of forms 

 which attain their maximum development in warmer seas. 



The Stationary Diatoms too attain their greatest development 

 at the same seasons, and can then cover the algae of the littoral 

 with a close brown felt. 



All these forms are undoubtedly stationary. As crusty cove- 

 rings to stones and rocks, they lie dormant during the unfavour- 

 able seasons. 



It appears to us to be most natural to assume the like in 

 respect to the Neritic Diatoms, when we know that they have 

 the power of forming spores which fall to the bottom. Through 

 PETERSEN'S investigations - 'n the Lim Fjord we have besides proof 

 that the Neritic Diatoms may be stationary. 



CLEVE'S Type 2 (Ns), is composed of forms, which, in his opinion, 

 come from the West Coast of Norway. He, however, says ([97] 

 pp. 7, 8) that he is not from personal experience, acquainted with 

 the Plankton of the Norwegian coast, but only draws his conclusions 

 from the appearance of the species in the Skagerak. CLEVE does 

 not give any reason on which to base this assumption, which is, 

 in any case, erroneous, as none of those species are typical of the 

 Norwegian West Coast. They are all to be met with there at 

 various seasons, but only as subordinate constituents of the Plankton. 



This can be seen from our Tables A, 3 & 5) and from 

 CLEVE'S later researches, mentioned in PETTEKSSON and EKMAN'S 

 work [98]. 



The typical forms on the West Coast of Norway are, as we have 

 already indicated, on the whole the same as those of the Skagerak. 

 As is usually the case with the fauna and flora of neighbouring 

 coasts, the Neritic Plankton in both places contains most of the 

 species in common, and the difference mainly consists in the rela- 

 tive numbers of the species being altered. The 4 species that 

 are named are at least as common in the Skagerak as on the 

 West Coast, and there, therefore, exists no reason for withdrawing 

 them from the others and placing them in a separate group. 



This theoretically constructed Plankton group must, therefore, 

 in any case in its present form, be abandoned. If it is to be 

 replaced by any other, the West Coast Flora must be divided into 

 many communities, as the Plankton there, just as in the Skagerak, 

 changes with the seasons. 



Such a division we however consider to be of little consequence 

 so long as we do not know more of the development and life of 

 the Neritic Plankton. 



CLEVE'S other three types, the Arctic Neritic Plankton, Con- 

 cinnus-Plankton and Halosphffira-Plankton we will treat of shortly. 

 The first is a well defined community of high Arctic forms which 

 vegetate during winter along the whole of our Northern Ocean 

 shores (southern limit at Stadt), and which during summer are but 

 met with on the coasts of Greenland and Spitzbergen. 



The Concinnus-Plankton and Halosphsra-Plankton are pre- 

 sumed to belong especially to the North Sea. We do not possess 

 sufficient materials to form an opinion on the correctness or other- 

 wise of adjudging these to be separate communities. 



The Coseinodiscus concinnus we have deemed to be Oceanic 

 on account of its close relationship to C. oculus iridis. From G. 

 MURRAY'S investigations, which we unfortunately are only acquainted 

 with from the preliminary treatise in the Fishery Boards Report 

 [97], it appears to have spores, so that it may be regarded as 

 truly Neritic. 



Halosphcera too, appears to possess a sort of cyst formation*), 

 which, however, has not been more fully investigated. Its range 

 is very great, being met with from the Mediterranean, where it 

 was first discovered, to the Lofotens, and in to the very quays of 

 Christiania. 



We cannot here enter more fully into the various forms of 

 the Neritic Plankton. If these organisms are to be divided into 

 communities, one would have to arrange the species in groups, 

 according to each stretch of coast, requiring similar conditions of 

 existence, and attaining their maximum at similar seasons. In order 

 to do this we have yet too little material to go upon, but we do 

 not doubt that this can be done, and, when it is accomplished, 

 that, by comparing the development of the corresponding groups at 

 the various places, we should obtain important contributions towards 

 comprehending the climate of the ocean. 



The Neritic Diatom Plankton can, to still less an extent than 

 the Oceanic, give any basis for hypotheses concerning the flow of 

 waters from distant parts. 



The Sira-Plankton disappears, for instance, from the shores 

 of Greenland in September, appearing in the Skagerak first in 



*) Shortly described in a preliminary note by CLEVE. We also have 

 occasionally met with such formations. 



