310 REPORT OF NATIONAL MUSEUM, 1892. 



Two extremes of rate of evolution are exemplified by the fresh-water 

 mollusca on the one hand and the placental mammals on the other. 

 The evolution of the former has been slower than that of any other 

 animals of equal biological rank, and the evolution of the latter more 

 rapid than that of any others, even among- their contemporary forms 

 of life. 



Applying the foregoing statements to the methods which have been 

 adopted in the construction of the geological scale and in the recogni- 

 tion of its divisions, we observe that for the portion which may reason- 

 ably be assumed to represent fully the first half of geological time the 

 work has been accomplished entirely by means of marine invertebrate 

 fossils. These only were available for that purpose, but they were 

 sufficient. We further find that the remainder of the Paleozoic portion 

 of the scale was constructed by means of marine invertebrate remains 

 with only the auxiliary aid of plant and fish remains. The latter aid 

 was not really necessary, because the succession and completeness of 

 Paleozoic marine life was unbroken, and the remains which those forms 

 of lite afforded constitute of themselves abundant material for charac- 

 terizing consecutive divisions of the scale. 



The opinion formerly prevailed among geologists that at the close of 

 Paleozoic time there was a material and general break in the succes- 

 sion of marine life coincident with that which was assumed to have 

 taken place in the case of land plants, and which was also assumed to 

 have been at least approximately coincident with the introduction of 

 dinosaurs, birds, and nonplacental mammals. 



It is now known, however, that the succession of marine invertebrate 

 life was as complete from Paleozoic to Mesozoic time and from Mesozoic 

 to Cenozoic time as it was for any other portions of the geological scale. 

 Furthermore the remains of Mesozoic and Cenozoic marine inverte- 

 brates are as sufficient for the characterization of those divisions of 

 the scale as are remains of marine invertebrates for the Paleozoic por- 

 tion. 



It is thus apparent that there has been a continuous and full succes- 

 sion of marine invertebrate life through the whole time range of the geo- 

 logical scale, and that its remains are as sufficient as any fossil remains 

 can be for the characterization of ever} 7 one of its divisions wherever 

 they are represented by fossiliferous rocks of marine origin. Further- 

 more, from the beginning to the end of geological time, there has been 

 a good degree of uniformity of the rate of development of marine inver- 

 tebrates, and the reciprocal relation of the various kinds to one another, 

 so far as concerns its bearing upon geological questions, has always 

 been intimate and of a comparatively uniform character. Therefore, the 

 paleontological record which they have produced is to a great degree 

 complete in itself and harmonious in all its parts. 



On the other hand it is apparent that the longest known time range 

 of the fossil remains of any of the other kinds of animals or of land 



