316 REPORT OF NATIONAL MUSEUM, 1892. 



relative value in the identification of formations. Naturally the re- 

 mains of aquatic faunas exceed all others in value for this purpose 

 because the faunas not only found a congenial habitat in the waters 

 in which were deposited the formations that now contain their fossil 

 remains, but they could have existed in no other. Their whole life 

 history, with the minor exception, of migratory fishes, especially such 

 as entered nonniarinc waters, was therefore intimately and wholly con- 

 nected with the production of those formations. 



A large proportion of all the formations which are characterized by 

 the remains of aquatic faunas contain none of land faunas and floras, 

 but in other formations remains of the latter kind are found com 

 mingled with those of aquatic faunas. In all such cases the remains 

 of land faunas and floras reached their intombment by accidental 

 means while the intombment of the remains of aquatic faunas was a 

 natural result of the character of their habitat. Moreover, all the mem- 

 bers of extinct aquatic faunas which possessed fossilizable parts are 

 likely to have been represented by fossil remains, because in their in- 

 tombment they were not separated from their habitat, while the intomb- 

 ment of all remains of land faunas and floras was not only accidental 

 but necessarily partial as regards the faunas and floras from which 

 they were derived.* 



Again, the existence of every extinct aquatic fauna had not only an 

 intimate connection with the conditions which produced the formation 

 in which the remains are found, but it began its existence as a fauna 

 with the establishment of those conditions and was extinguished as 

 such when the conditions were changed, and largely or wholly in conse- 

 quence of the change. It might easily, and evidently often did, happen 

 that changes of physical conditions which caused the extinction of one 

 aquatic fauna and the introduction of a succeeding one would not mate- 

 rially affect the continued existence of the fauna and flora of the adja- 

 cent land which were contemporary with the extinguished fauna. In 

 such a case the land fauna or flora began its existence before, or con- 

 tinued it after, that of the aquatic fauna, or its existence may have 

 extended continuously from the epoch before to that after the one in 

 wlrch the aquatic fauna lived. In such a case also, while the aquatic 

 fauna was characteristic of only one formation and one epoch, the land 

 fauna and flora may have characterized two or three formations and as 

 many epochs. Furthermore, geologists sometimes find evidence from 

 the association of their remains with those of aquatic faunas that land 

 faunas and floras were repeatedly and materially changed during a 

 period within which aquatic faunas of the same region suffered com- 

 paratively little change. 



It is true, as mentioned in those paragraphs of essay II, in which 

 methods of defining and characterizing formations are discussed, that 



*See pages 254-261. 



