94 



Wilhelm Björck 



of Anholt; the upper strata of the water are on the other hand subject to conside- 

 rable changes in these respects, in addition to which the degree of saltness of the 

 Baltic water in Öresund is lower than in the surface layers of the Kattegat. Pro- 

 bably in the first place on account of the smaller variation oP temperature in the 

 lower strata of the water (see p. 78) a number of species can be met with in Öre- 

 sund and the south-east of the Kattegat at an essentially smaller depth than is 

 usual within the open Scandinavian sea-region. Likewise the unfavourable conditions 

 in the surface strata drive a number of otherwise littoral forms to find a lower place 

 in the sub-littoral regions. A concentration of the vertical regions has in a measure 

 taken place. 



5) The composition of the fauna lends no support to the hypothesis, put for- 

 ward by Lönnbekg, of the persistence of an arctic character in the fauna. The forms 

 existing in the boreal and atlantic-mediterranean regions constitute the majority, 

 and of those existing in arctic or boreo-arctic regions only three are met with in 

 Öresund that have not been observed on some part of the coast of the British Isles 

 or in the southern North Sea. Probably the crustacean fauna, as has been shown to 

 be the case in the Kattegat as regards molluscs, shows relatively a somewhat larger 

 number of arctic-boreoarctic-boreal forms and a somewhat smaller number of 

 atlantic- boreal forms than the southern North Sea, but this fact finds its natural 

 explanation in the greater ease with which the forms coming from the north could 

 enter the Skager Rak along the Norwegian channel and pass on through the eastern 

 Kattegat to Öresund. 



