280 REPORT OF NATIONAL MUSEUM, 1892. 



of practical importance in all such investigations. Besides other appli- 

 cations that may be made of the facts mentioned in those remarks, they 

 show the necessity for the study of the different kinds of aquatic faunas 

 with relation to one another, and make it evident that so great a differ- 

 ence in their kinds and in the conditions of their origin implies a wide 

 range of practical applicability to geological studies. 



Comparative studies of this kind are prosecuted mainly by the philo- 

 sophical method and require a consideration of various biological and 

 physical facts. Among them are those which relate to the various gen- 

 eral conditions under which sedimentary formations were produced and 

 the more special conditions under which the aquatic faunas lived whose 

 remains now characterize them. The general conditions referred to 

 are largely of a geographical character, while the more special relate 

 to the quality of the water in which the respective faunas lived as the 

 element of their habitation. 



The fact that the fossil remains of aquatic animals generally possess 

 inherent and unmistakable evidence as to the character and quality of 

 the respective bodies of water in which were deposited the sedimentary 

 rocks which are now found to contain them was recognized at an early 

 date, and the character of that evidence is such that there never has 

 been any important disagreement among geologists as to its trust- 

 worthiness.* Indeed they usually ami properly assume that there is as 

 little room for reasonable doubt as to the quality of the water in which 

 each fossil aquatic fauna lived as would be the case if those waters 

 were still subject to a gustatory test or to chemical analysis. t Admis- 

 sible evidence as to the quality of the water relates only or mainly to the 

 presence and comparative proportion of salt in, or to its absence from, 

 the various bodies of water which have existed during geological time, 

 ami in which sedimentary deposits were made. In other words, it is 

 the kind of evidence that indicates whether those waters were fresh, 

 brackish, or of marine saltness. Such evidence indicates whether the 

 water in which a given formation was deposited was marine, estuarinej 

 lacustrine, or tluviatile, and all this evidence, although relating to 

 physical questions, rests upon comparisons of fossil remains of aquatic 

 faunas with corresponding parts of members of now living faunas whose 

 structural characteristics and restrictions of habitat are known. The 

 marine so greatly preponderate over all other kinds of aquatic faunas 

 that it is convenient in discussing the sedimentary formations to make 

 the general distinction of marine and nonmarine, the former term not 



'The criteria of the character of formerly existing bodies of water are discussed 

 in Essay vn, hut they are briefly referred to here in discussing tin- subject of this 

 essay. 



tit is probable that the earliest oceanic waters were much less salt than are those 

 of the present day because the earth has ever since been subjected to a process <>f 

 leaching, with the oceans as a reservoir, but the comparisons here made apply 

 mainly to comparatively late portions of geological time. 



