334 REPORT OF NATIONAL MUSEUM, 1892. 



It is apparent from the foregoing remarks that upon physical evi- 

 dence alone it is not practicable to satisfactorily classify the sedimen- 

 tary formations of the earth in such a manner as to serve the purpose of 

 thorough geological investigation. Therefore such data are in this, as 

 in most other cases, chiefly valuable as being accessory to the evidence 

 afforded by biological data. 



The biological criteria which are relied upon by geologists to distin- 

 guish from one another the sedimentary formations which have been 

 produced in marine waters, or in those of inland seas, lakes, rivers, or 

 estuaries, relate to the characteristics of faunas which now inhabit 

 those waters respectively, and to the differences from one another of 

 such faunas. That is, the conclusions which geologists reach concern- 

 ing the questions just indicated are based upon now-existing phys- 

 ical conditions, upon the known character, structure, and habits of 

 animals with relation to those conditions, and upon the assumption 

 that in past geological epochs animals of a given character and struc- 

 ture had similar habits, and lived under conditions similar to those 

 which are congenial to their living congeners. 



To aid in defining these criteria it is necessary to review the animal 

 kingdom as it now exists, and to select for consideration those portions 

 of it which furnish data upon which to base the necessary definitions. 

 This selection is based mainly upon the function of aqueous respiration, 

 because it is only animals possessing this function that have a direct 

 relation to the character of the water in the sediments of which their 

 remains may become intombed, and because these sediments and their 

 organic contents are similar in their origin to those which constitute 

 the fossiliferous formations with which the geologist has to deal. Land 

 animals are only briefly referred to in this review because they have at 

 best only incidental relation to the character of the respective bodies 

 of water near which they live and to the sediments which are depos- 

 ited in them, and for a similar reason plants also will be only briefly 

 considered. Still, remains of land animals and plants have an indirect 

 value in this connection. For example, it is obvious that such remains 

 are more likely to find intombment in inland than in marine waters. 

 We also may assume that they rarely reach those of the open ocean or 

 that they quickly become destroyed by the triturating action of the 

 waves if they reach oceanic waters. 



All those aquatic animals whose bodies possess no internal or exter- 

 nal skeleton, or such portions as resist decomposition after death, are 

 also excluded or only incidentally mentioned, because it is those parts 

 only that really become fossilized, as has already been explained ir 

 Essay t, and also because in the application to paleontological inves- 

 tigation of the facts to be presented in this review reference can be 

 made only to the fossil remains of animals similar to those now living. 

 All extinct animals are also excluded from this review because it is 

 these to which the criteria based upon living forms are to be applied. 



