GENERAL CHARACTERISTICS OF THE NORTHERN SEAS 



67 



Fig. 17. 1 Northern boundary of propagation of the bivalve Portlandia arctica 

 (brackish-water province) ; 2 Southern boundary of propagation of the bivalve Pec- 

 ten (Propeamusium) groenlandicum (marine province, according to Gorbunov, 

 1941); 3 Northern boundary of propagation of brackish-water plankton Crustacea 

 Limnocalanus grimaldi and Dropanopus bungei (Bogorov, 1944). 



Hence on the whole the following scheme of zoogeographical subdivision 

 is obtained for the Arctic region (Fig. 16): 



The echinoderm group, as one of the best studied, may be cited as an 

 example of the zoogeographical analysis of the Arctic basin fauna. According 

 to A. M. Djakonov (1945) the following* groups can be distinguished among 

 the 121 species of Echinodermata known in the Arctic Ocean. 



(i) Cosmopolitan species, 4 per cent 

 (ii) Boreal species, immigrants from the Atlantic, 23 per cent 



* Species were not taken into account when the percentage (23, 28 and 45) were cal- 

 culated. 



