150 NEW YORK STATE MUSEUM 



nordmanni, which occurs more and more frequently in pro- 

 portion to the increase in the salinity of the water (westward). 

 In descending south the E v a d n e s tend more and more to replace 

 the Bosminas, but the latter have been found even as far 

 down as Kiel. Another abundant euryhaline crustacean is P o d o n 

 intermedius. From the point of view of the pelagic fauna, 

 the Gulf of Finland may be compared to a lake broadly opened 

 on the Baltic (salinity of .73 permille at Cronstadt; 2.62 permille 

 at Seskar). Along the extent of Gotland, the marine crustacean 

 fauna is found in the Baltic as far as Kalmar sound between Oland 

 and Sweden. One interesting copepod found isTemora velox, 

 known for a long time as an inhabitant of brackish waters. This 

 species appears to have adapted itself in a special manner to extreme 

 conditions of existence in the Baltic, for it has spread out every- 

 where there and is so abundant as to play an important part in the 

 nourishment of certain fish. 



A change similar to the above has been found among the Mollusca. 

 Species of L i m n a e a , such as L. palustris and p e r e g e r, 

 replace the L i 1 1 o r i n a species. When the salinity of the water 

 is low along the coast the two forms are found living together, and 

 with them, is also found a river form, Neritina fluviatilis. 

 Common forms of Planorbis and B y t h i n i a have been 

 enumerated from the Baltic in addition to the Limnaeas and 

 Neritinas. In the Gulf of Bothnia many of the common 

 air-breathing pond snails have habituated themselves to the slightly 

 saline waters of that part of the Baltic (Forbes, p. 90, 231; O'Con- 

 nell, p. 71). 



There is a very rapid decrease eastward in the number of species 

 comprising the whole fauna. Mobius (1872, p. 279; 1873, p. 138) 

 describes the Baltic as being faunistically divided into two basins, a 

 western and an eastern ; the former marked by a rich fauna, the latter 

 by a strikingly impoverished one. In his earlier report on the Baltic 

 fauna (1872, p. 277), he gives the total number of observed inverte- 

 brate animals as amounting to about 200 species (exclusive of infus- 

 orians and crinoids) , only one-fifth of which were found in the eastern 

 basin of the Baltic which begins between Rugen and the southern 

 extremity of Sweden. In the later report (1873), this number 

 is increased to 241 species for the western basin (exclusive of 

 infusorians, rhizopods, ostracods) of which 69 have been found in 

 the eastern. The following table (after O'Connell, p. 72) shows 

 the rapid decrease in the species of the Baltic fauna and gives a 

 comparison of that fauna with a normal marine fauna: 



