360 REPORT OF NATIONAL MUSEUM, 1892. 



various as in marine waters. It is also true that molluscan life is 

 often locally abundant in shallow fresh waters, but, as already several 

 times mentioned, the variety is extremely meager. All these peculiar- 

 ities are distinctly observable among- the fossil faunas of the non- 

 marine formations. 



Other general indications of difference between marine and non- 

 marine formations are furnished by remains of laud plants and animals. 

 Open-sea formations are naturally free from any vegetal remains de- 

 rived from the land, although coal and other materials of vegetal origin 

 are not unfrequently found alternating with layers containing marine 

 fossil remains. These, however, as explained on a preceding page, are 

 regarded as cases of emergence of the bottom of shallow sea waters and 

 the subsequent subsidence of the same as plant laden marshy land. It 

 is a matter of fact, the reason for which has been suggested in preced- 

 ing essays, that plant remains of any kind, especially such as are in 

 a classifiable condition, have so rarely been found associated with re- 

 mains of denizens of marine waters that the discovery of fossil plants 

 in any formation is of itself presumptive evidence of its nonmarine 

 origin. 



It has already been shown on preceding pages that the remains of 

 land animals have so seldom reached marine waters or, having reached 

 them, they were probably so generally destroyed by the triturating 

 action of coast waves that the discovery of any of this kind of fossil 

 remains in any formation may also be regarded as presumptive evidence 

 of its nonmarine origin. 



The foregoing statements have been made with reference to indica- 

 tions which are either of a general character or without direct relation 

 to the quality of the waters in which sedimentary formations have been 

 deposited. All the direct evidence, as already has been fully stated, 

 is derivable from the fossil remains of the denizens, especially the gill- 

 bearing kinds, of the waters in which were deposited the formations' 

 under investigation. 



Referring to the foregoing review of the animal kingdom, including 

 the tables which it embraces, it will be seen that a large number of 

 families of both fishes and invertebrates are confined to a marine hab' 

 itat, and that every member of even some of the higher divisions is ; 

 similarly restricted. For example, every known member of the classes' 

 Cephalopoda and Brachiopoda is confined to a marine habitat. It will! 

 also be seen that a certain small number of families, especially of the' 

 mollusca, are equally restricted to fresh waters. The significance of ' 

 such cases ns these lias already been pointed out, but it is desirable to> 

 refer to them again. 



Fossil remains representing any one of these kinds of animals may 

 be taken as positive evidence of the quality of the water in which was 

 deposited the formation containing them, provided there shall be no 

 room for reasonable doubt that the animals were really denizens of 



