BUFFALO SOCIETY OF NATURAL SCIENCES 77 



there alive or dead, and their hard parts will be preserved with 

 those of the typical marine species. 



2. The number of genera and species in fresh water is as a rule 

 very much smaller than that in the neighboring sea, although the 

 number of individuals may be nearly as large. Entire classes of or- 

 ganisms are wanting, while other classes are represented by only a 

 few genera and species. Fresh water organisms are distributed by 

 rivers and are found living in lakes and lagoons on the subaerial por- 

 tions of deltas, in lakes, playas and more rarely epicontinental seas, 

 all of which water bodies are geologically short lived. 



3. Brackish waters may be considered under two types: 



(a) The brackish waters or land-locked epicontinental seas such 

 as the Baltic show a range in salinity from that of normal sea water 

 to that of the rivers which empty into such water bodies. The fauna 

 is made up of a modified marine and a modified fresh-water fauna, and 

 always has its nearest relatives in the open sea, on the one hand, and 

 in the rivers and connecting fresh-water lakes, on the other. Since 

 the species contributed by the marine waters are so much more num- 

 erous than those contributed by the rivers, the greater number of 

 the species in the brackish water will be related to marine forms. 



(b) The brackish waters of estuaries are not stable enough to 

 have a fauna which may be considered endemic. The marine fauna 

 lives along shore and in the waters not too much disturbed by the 

 tidal current, but the organic remains found in the estuaries are ma- 

 rine. Moreover, along the coasts affected by the deposition of the 

 tidal muds, no organisms live and it is only many miles away from the 

 estuary proper that the marine forms are found whose hard parts, 

 carried by the tidal currents and in time comminuted, finally come 

 to settle on the floors of the estuaries and on the river banks. These 

 hard parts are carried up the estuaries as far as the tidal current is 

 felt and it is only above this point that the fresh water forms are 

 found. In the very small area between the purely fresh and the domi- 

 nantly marine waters is the brackish-water area in which there may 

 be a small mixed fauna. 



APPLICATION TO THE PAST 



Armed now with a considerable wealth of facts drawn from the 

 present, we may turn to the past in an attempt to set forth any 

 available criteria which- may be used in determining the nature of a 

 given habitat. 



