REPORT OF THE DIRECTOR I920-2I 



155 



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Fig. 1 Sketch map of Baltic showing permillage variations in salinity 

 (after O'Connell, 191 6). 



Corresponding to these changes in salinity are certain very definite 

 changes in the fauna." (O'Connell, p. 70; see also Grabau and 

 Jacobsen) . 



As the salinity of the water decreases from that normal for sea 

 water, the fauna changes from one typically marine to one in which 

 only a few marine groups are represented and finally to a fresh- 

 water fauna. Each phylum is affected. 



Pouchet and de Guerne (p. 919-21), from a study of dredgings in 

 the Baltic, reported from the Gulf of Finland a crustacean fauna 

 made up almost entirely of fresh-water types. One of the types, 

 Bosmina longirostris, shows a great abundance of 

 individuals, for it represented by itself alone three-fourths of the 

 mass of the animals obtained at the various stations. Associated 

 with the fresh-water types is a marine pelagic form, E v a d n e 



