On the Fossil Shells of the Paris Basin. 209 



more from our own, belonging frequently to extinct races of 

 animals ; and it is necessary to be able to employ these induc- 

 tions, so as to verify each step by observation. Thus for 

 example, after obtaining the first results of comparison be- 

 tween the fossil and living species, the former must then be 

 compared with the fossils of different types, and on this pro- 

 cess being extended to the whole series of fossil shells, our 

 judgment is to be formed from the result. It is by following 

 those steps of which we are now about to afford a rapid 

 view, and which we have endeavoured to detail at large in 

 this work, that we may hope to facilitate a knowledge of the 

 geology of a set of strata, which will serve as a starting point 

 in the study of tertiary rocks, and at the same time present 

 to the zoologist interesting facts relative to species which 

 no longer exist on the surface of the earth. 



From the study of our species of the Paris basin being 

 nearly complete, they afford the hope, after very extensive 

 researches on the subject, that we shall be able to deduce 

 , from them a standard of comparison for the study of other 

 tertiary beds in which the same fossils occur. 



In a work presented to the Academy of Sciences in 1831, 

 we have given the results of the comparisons which we had 

 made between the shells of living species and the fossil 

 shells of the tertiary deposits of Europe. One of the princi- 

 pal results of this comparison has been the determination of 

 the peculiar characters of these beds, and an indication of their 

 superposition ; these results with prophetic spirit appear to 

 have anticipated subsequent acquisitions which have been 

 made to science by the researches of geologists, and it is thus 

 I that we have also realised our former conjectures, and have 

 established the importance in these pursuits, of inductions 

 derived from zoological inquiries. 



Another result obtained by the same means is, that no 

 one species of shell has been found to belong to both 

 secondary and tertiary strata: thus the upper beds of the 



