358 REPORT OF NATIONAL MUSEUM, 1892. 



different as regards palustral plants, the character of most of wliich 

 may usually be accepted as trustworthy evidence of their nonmariue 

 habitat. Still, the remains of land plants, like those of land animals, 

 are far more likely "to have found the quiet entombment necessary to 

 their preservation in the sediments of nonmarine than in those of 

 marine waters, because the former waters were surrounded by the land 

 upon which the plants grew, and because the nonmariue sediments 

 receiving such remains are, as a rule, not subject to the destructive 

 littoral wash which usually prevails along sea borders. 



The following facts and assumptions have a direct bearing upon the 

 foregoing statements and discussions and upon their practical applica- 

 tion to geological investigation. 



The various bodies of water which existed during geological time, 

 and which constituted the habitat of aquatic animals, were of the same 

 kinds that now exist, namely, marine and fresh, together with those of 

 the various intervening grades of saltness. Although it is probable 

 that the marine waters of early geological time were not so salt as 

 those of the present oceans, it is believed that this difference in salt- 

 ness has not been so great as to make any appreciable difference as to 

 legitimate conclusions of the kind that have been indicated on pre- 

 ceding pages. It seems to be especially evident that this difference 

 has been thus inappreciable since the close of paleozoic time, since 

 which time the greater part of the known unmistakably nonmarinc 

 formations were deposited. 



Existing bodies of water are constantly depositing materials similar 

 to those of which the sedimentary rocks are composed. 



In past geological epochs the habits of animals of a given character 

 and structure were the same as those of similar now living animals, 

 and they lived under conditions similar to those which are congenial 

 and necessary to their now living congeners. Also in those epochs 

 plants of a given character lived under conditions similar to those 

 which are necessary to the corresponding kinds of now living plants. 



Those animals alone which are furnished with organs for aqueous 

 respiration can be confidently relied upon as indicating the character 

 of the water in which they respectively lived. 



Thus, if all the known now living members of a given family are con- 

 lined to marine, or to fresh waters, as the case may be, it is assumed 

 that the habitat of the extinct members of such families were similarly 

 restricted, and that the presence of fossil remains of such animals in a 

 given formation is, in the absence of conflicting facts, sufficient evi- 

 dence of its marine origin on the one hand or of its fresh-water origin 

 on the other. Again, if a given family is known to have representa- 

 tives now living in marine, brackish, and fresh waters, respectively, it 

 is assumed that it had a similar range of habitat during past geolog- 

 ical epochs. Therefore, the discovery in a given formation of fossil re- 

 mains of a single representative of a family having such a varied range 



