104 New York State Museum 



which are of 'economic importance, this optimum salinity 

 has been determined. 



The Baltic sea is a noted area for studying the influ- 

 ence of a diminished salt content upon the animal life. 

 It shows a very striking decrease in salinity eastward and 

 in a large way the responses of the fauna to it. The 

 North sea has the normal marine salinity of 35 parts per 

 thousand of salt which decreases steadily going eastward 

 in the Baltic until at the northern end of the Gulf of 

 Bothnia the water is practically fresh. 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. From the Gulf of Finland is re- 

 ported a crustacean fauna made up almost entirely of 

 fresh-water types, one of them showing such a great 

 abundance of individuals that it alone represented three- 

 fourths of the mass of the animals obtained at the various 

 stations. With these fresh-water types occurs a 

 marine pelagic crustacean that becomes more and more 

 abundant westward with increased salinity. A brackish- 

 water type also occurs here and this species has so 

 adapted itself to the extreme conditions of existence in 

 the Baltic that it has spread out everywhere and is so 

 abundant as to play an important part in the nourishment 

 of certain fishes. The Gulf of Finland with its extremely 

 low salinity may be compared to a lake broadly opened 

 on the Baltic. Among the Mollusc a of the Baltic a change 

 similar to the above has been found. Brackish and fresh- 

 water snails and even a river form are reported from 

 the waters of low salinity, and in the Gulf of Bothnia 

 many of the common air-breathing pond snails have ac- 

 customed themselves to the slightly saline waters of that 



