312 REPORT OF. NATIONAL MUSEUM, 1892. 



North America, and the same species have been found in an overlying 

 formation which is characterized by an abundant placental mammalian 

 fauna. Such a survival of gill-bearing mollusks implies the continua- 

 tion of a congenial aquatic habitat, continuous sedimentation, and a 

 continuous record of time. In this way specific molluscan forms are 

 found to have bridged the gap between characteristic mesozoic, and 

 equally characteristic ceriozoic vertebrate forms so far as the existence 

 of the latter has been proved by the discovery of fossil remains. It is, 

 therefore, e\ ident that neither the exceptionally rapid rate of evolution 

 like that of the placental mammals, nor the exceptionally slow rate, 

 like that of the fresh water mollusca, can be used independently as a 

 standard of geological time. 



It has been shown on preceding pages that it is the general advance- 

 ment in biological rank for all organic forms and for the whole of geo- 

 logical time that constitutes the ideal ultimate standard of measure 

 for that time. It does not necessarily follow, however, that the geo- 

 logical scale is actually based upon the combined average rate >f 

 advancement of all those forms because this is a factor which can not 

 be definitely ascertained. Still, in all cases it is necessary to apply 

 that idea so far as is practicable. 



In view of the facts recorded in the preceding paragraphs we must 

 necessarily place the highest estimate of chronological value upon the 

 fossil remains of those kinds which have existed under the most nearly 

 uniform conditions through the whole of geological time, and which 

 give evidence of the most nearly uniform advancement in biological 

 rank". Accordingly the remains of marine invertebrates possess legit- 

 imate claims to a higher estimate of chronological value than do those 

 of any other kinds of animals or of plants. 



It is true that the rate of development in biological rank of marine 

 invertebrates does not embrace i he entire advance for the whole animal 

 kingdom because it begins in the scale as it is now known with many 

 highly organized forms and ends without including the vertebrates, 

 but this fact does not affect any of the necessary elements of their 

 superior chronological value which have just been mentioned. The fol- 

 lowing summary of facts relating to the marine invertebrates show their 

 principal claims to the highest estimate of value in characterizing the 

 divisions of the geological scale and in determining the geological ago 

 of the strata in which their remains are found. 



The marine invertebrates embrace five of the six subkingdoms or 

 branches of the animal kingdom. 



They have coexisted in every stage of geological time while the 

 known time-range of other animals, as well as of land plants, has been 

 very much less. 



The preservation of their remains having been a natural consequence 

 of the character of their habitat they are faunally more complete than 

 are those of any land animals, and for the same reason they are florally 

 more complete than are remains of land plants. 



