FACTORS GOVERNING DISTRIBUTION 1045 



is 34 permille in the Skagerak, 22 permille in the Kattegat, 6 per- 

 mille at Riga and 3 permille at the northern end of the Gulf of 

 Bothnia (Ulelborg), The vertical range is also greater than in the 

 open sea, the salinity in the "Great Belt" of the Baltic increasing 

 downward from 10 permille on the surface to 30 permille at the bot- 

 tom (66 meters). 



Stenohalinity and curyhalinity. Many animals cannot live in 

 water with less than 30 or 35 permille of salts; i. e., that of the 

 normal open sea. These stenohalic * types will die when the sal- 

 inity is lowered or raised. Euryhalic j types, on the other hand, can 

 suffer a considerable freshening" of the water, and will live as long 

 as any salt remains. Brackish-water organisms are adapted to a 

 small amount of salt and will suffer if the amount is increased. The 

 brackish state of water has never been definitely delimited, but prob- 

 ably a salinity of 2 or 3 permille (.2 to .3 per cent.) would be the up- 

 per limit. Of other mineral matter besides the normal salts iron in 

 the form of ferrous carbonate (FeCOg) may sometimes be present 

 in considerable quantity, especially in more or less land-locked 

 portions. Such excess of iron or of other minerals in solution 

 appears to have a distinct effect on the growth of the fauna living 

 in the water, resulting in its dwarfing, as has been repeatedly found 

 by experiments. As an example may be mentioned the fauna of 

 the Clinton iron ore layer of Rochester, which consists of individuals 

 having "an average of about one-third the diameter of the same 

 species in the beds just above and below" ( Loomis-28 : 5p5). The 

 fauna of the Tully pyrite layer of western New York, consisting of 

 upward of forty-five species, is composed of individuals "on an 

 average only one-fifteenth the size of the same species in the normal 

 and preceding Hamilton fauna." ( Loomis-28 :9.?o.) In this case 

 the dwarfing was undoubtedly in part effected by the HoS liberated 

 by sulpho-bacteria from decaying organic matter. The reaction of 

 this upon the iron carbonate produced the pyrite which enclosed 

 the fauna. (FeC0,+2HoS+0 = FeS,-t-C0,+2H,0.) 



Quantitx of Air. The quantity of air in the water, both salt 

 and fresh, depends upon the depth and the amount of agitation. 

 Deeper strata of water have necessarily less air than those nearer 

 the surface, while agitated water will include more air than still 

 w^ater. It is upon the oxygen of this included air that all animals 

 are dependent, and when it is present in insufficient quantity the 

 animals generally perish, though some will come to the surface for 

 more air. In partially enclosed bodies of water where the density 



* From (TTecos = narrow and &\s = the sea, (salt,'. 

 t From eiipvs = broad. 



