Handbook of Paleontology 105 



part of the Baltic. There is a rapid decrease eastward 

 in the number of species comprising the whole fauna, so 

 much so that this area has been described as being divided 

 faunistically into two basins, a western and an eastern, 

 the former characterized by a rich fauna, the latter by a 

 strikingly impoverished one. About 240 species have 

 been reported from the western basin and of these species 

 only about one-fifth are found in the eastern basin. With 

 decreasing salinity dwarfing appears. The animals of the 

 eastern basin are more dwarfed than those in the western 

 basin, and the best examples are found among the mol- 

 lusks, in which group, in addition to being dwarfed, the 

 shells become poor in lime. Two good illustrations are 

 the Common Mussel (Mytilus edulis) and the little 

 Macoma balthica (groenlandica) so characteristic of 

 northern waters. Another noteworthy case of dwarfing 

 is exemplified by the common European Cockle (Car- 

 dium edule) which has a large, rough, thick shell and 

 thrives best under purely marine conditions. In normal 

 conditions in the North sea this Cockle is the size of an 

 apple ; at Stockholm where the water shows only ten parts 

 per thousand of salt, the shell is the size of a walnut; 

 with decreasing salinity eastward in the Baltic, the shell 

 at Konigsberg reaches the size of a hazelnut, and at Reval 

 only the size of a pea. A very important fact brought 

 out in the studies of the Baltic sea is that however 

 dwarfed or otherwise modified the species may be the 

 marine forms of the Baltic are not different specifically 

 from those living in water of normal marine salinity, nor 

 do the fresh-water forms differ specifically from those 

 found in the rivers emptying into the Baltic or those in 

 near-by fresh-water bodies. 



