286 FOSSIL OSTREID^ OF NORTH AMERICA. 



cannot live in waters that are not more or less saline. It is a fact, well 

 known to the trade, that great destruction of the oyster beds is liable 

 to occur at and near the mouths of rivers during times oLhigh and long- 

 continued freshets in the rivers, by which an unusual quantity of fresh 

 water is passed over them. Lieut. Francis Winslow, U. S. N., of the 

 United States Fish Commission, has furnished me the following figures 

 indicating the range of siJccific gravity from distilled water to that of 

 the open sea, and also the minimum of that in which oysters will live. 



Specific gravity of open-sea Tvater 1. 027 



Specific gravity of the freshest waters in which oysters live 1.010 



Specific gravity of distilled water 1. 000 



In giving the minimum specific gravity of waters in which oysters will 

 live. Lieutenant Winslow does not mean to say that oysters immediately 

 die when placed in fresher waters, but he cites authorities to show that 

 the density of water cannot fall below 1.0 10 for any pro tracted period 

 without destroying the oysters that may have previously lived in it. 



It should be remarked that the specific gravity', as above given, is not 

 necessarily an absolute indication of the proportionate amount of com- 

 mon salt in the water indicated, but it is approximately so. There are 

 other substances held in solution in all sea and bay waters which, no 

 doubt, have much influence uiiou the molluscan life they contain ; but 

 common salt is so largely in excess of these, that it is usual to consider 

 that substance alone in such connection. 



The common living oysters of our coasts are not uufrequently found 

 in open sea waters, but their chosen habitat is in the waters of bays 

 and estuaries, which are of less than marine saltness. From the facts 

 here stated we see that there is a very considerable range of salfness of 

 the water in which oysters will thrive. 



In the case of the fossil Ostreidaj we cannot of course determine the 

 amount of salt the water contained in which they lived, but there are 

 certain circumstances attending thefossilization of those ancient oysters 

 that tell us with evident ai)2>roximatiou the degree of saltness which 

 characterized those waters. Such a judgment of the character of those 

 ancient and departed conditions is based upon our knowledge of the 

 habits of living mollusks in general, and those of the oyster in particular. 



For example, those oysters which are found living in open sea waters 

 are there associated, not with such mollusks as are its associates in 

 bays and estuaries, but with such as live only in the open-sea. If, then, 

 we find, as we often do, fossil oysters imbedded in strata, mingled there 

 with the remains of other mollusks which are closely related to such 

 living forms as are found only in the open sea, we necessarily infer that 

 the oysters in question had an open-sea habitat. Again, if we find, as 

 we often do, fossil oyster shells associated in the same strata with re- 

 mains of mollu.sks, whose nearest living relatives are found only in 

 brackish waters, we necessarily infer that those ancient oysters, like 

 their kindred which now exist, were capable of living in brackish as 



