MICROSCOnC MARINE ORGANISMS IN HYDROGRAPHY. 383 



distinguish one kind, characterised by Halu&phacra viridis, and occurring 

 in the winter; another by lihizosolcnia gracillima, occurring in the 

 summer ; one with Faracalanus parvus, and another with Pseudoccdanus 

 elongatus, and so on. 



In all cases it seems to be certain that the water containing this 

 first type is derived from the North Sea as well as from the Baltic. 



(2) The Didymus-2^lankton consists principally of diatoms, among 

 which the most characteristic species are Cliaetoceros curvicetus, Oh. 

 didymus, Ditylum BriijlitweUii, Rhizosolenia (data, and gracillima (the 

 latter probably a residuum of Type 1), Skeletonema costatum and 

 Thalassiothric Fraiienfddii (the latter probably common to Type 3). 

 A silicoflagellate, Dictyocha speculum, occurs constantly, but not 

 abundantly. The cilioflagellates, as well as the entomostraca, are 

 scarce. 



This kind of plankton was predominant in the Skagerack and 

 Kattegat in November, 1893, filling the fjords from the bottom to 

 the surface. With the water containing this kind of plankton the 

 herring arrived on the shores of Scandinavia. It seems to have been a 

 very large bulk of water that at this time set in to the coast, as it drove 

 away the whole of the summer water from bottom to surface. 



The diatoms of this type are not known from the Arctic Ocean 

 or from the Northern Atlantic, but are well known from the coasts of 

 France and Belgium and the English Channel, It seems thus to 

 be beyond doubt that the water came from the southern North Sea, 

 along the western coast of Denmark. The temperature, as well as the 

 salinity, were found to be variable, but the plankton constant. In the 

 Gullmarsfjord the water at the surface had a temperature of 7° C, at a 

 depth of 30 m. nearly 12°, and at the bottom only 4° to 5°. The 

 salinity amounted resjDectively to about 2G-27, 32 and 33 to 37 per 

 thousand. This variation may be explained by the mixture of the water 

 of the second type with the water previously present in the Kattegat. 

 Probably the warmest water was the most pure water of Type 2, and 

 corresponds to one of the kinds of water called by the Swedish 

 hydrographers the bank-ivater. 



(3) The third type of plankton, the Tricho-plankton, is distinguished 

 by its diatoms, especially the following species: Tlialassiosira loiujissima, 

 lihizosolenia styliformis, Chaetoceros atlanticus (in a less degree also by 

 Ch. horcalis and its variety Briylitioellii), and Biddulpliia muh'dcnsis. 

 The first-named species occur abundantly and almost pure in the 

 Northern Atlantic, south of Iceland ; the last-named I observed at 

 Plymouth, West Scotland, and in the North Sea. This plankton may 

 thus be considered a Northern Atlantic plankton. At the Scandinavian 

 coast it seems to occur very rarely in a pure state ; in fact, I have seen 



