328 ON THE DEPOSITS CALLED TERTIARY 



Cyclas, Unio, Tellina, Cardiurn, and Venus, some be- 

 longing to fresh and others to salt water, are found 

 promiscuously in the gulf of Livonia. Our own 

 muscles and oysters, and many more, thrive better in 

 fresh water than in salt; and reversely, many fresh 

 water shell fish can live in salt water, as those of salt 

 marshes are especially indifferent on this subject. 

 Independently of all thfese as grounds of doubt re- 

 specting such decisions, there is too much uncertainty 

 respecting the former states of the globe and of its 

 waters, to permit us to decide positively on the nature 

 of a stratum, from its fossils. Still less can that be 

 done by means of two or three species; nor are we 

 by any means sure that our anatomical arrangements 

 of living species as to this point, can be extended 

 retrospectively. If also we consider the repeated 

 changes of the surfaces, particularly of sea coasts, the 

 contests between salt water and fresh in aestuaries, 

 changes in the character of lakes in this respect, and 

 accidental transportations of shells themselves, it is 

 plain that there will always be danger of error in 

 forming absolute rules, and, still more, in attempting 

 to judge from a small number of circumstances or a 

 limited number of species. 



The difficulty is much increased where fossil species 

 are concerned, by the imperfections of their charac- 

 ters. 1'he minute parts on which the distinction so 

 often depends, such as the teeth of the hinge for ex- 

 ample, are frequently injured or destroyed. With 

 respect to tenderness and delicacy, or the reverse, 

 I may quote De France and Ferussac, as opinions 

 that there are no such general distinctions of character. 

 It is all matter of experience, or of empiricism; and 

 that experience also is limited. Nor can we decide 

 that an antient genus or species, now belonging to. 



