104 M. B. de Basterot's Observations on Fossil S/zdls. 



geological phasnomena. Curious resemblances will, I have 

 no doubt, be found to exist between these two distributions ; 

 and though fossil shells do not lead (as has been observed) to 

 equally exact results in solving the different problems relating 

 to the ancient zoology of our planet, with tlie remains of large 

 quadrupeds, they may, it appears to me, afford a firm support 

 for those deductions, which arise out of the examination of the 

 other classes. Shells are, moreover, indefinitely more abundant 

 than bones in the beds of the earth ; they are often the only 

 remaining documents to enlighten us respecting those changes 

 which have taken place, in the existence of living creatures 

 since the formation of the globe. Shells have, in fine, the ad- 

 vantage of being in a great measure found in those situations 

 which have given them birth, and thus remain the incontesti- 

 ble proofs of the presence and I'etreat of the sea. A striking 

 example of this will be seen in the fresh-water limestone, no- 

 ticed in the course of this memoir. These limestones, formed 

 in the first instance in waters filled with Limnece and Planorbes, 

 have been abandoned by them ; the sea has afterwards covered 

 them, and the inhabitants of that sea, Pholades, Saxicava;, and 

 other similar shells, have fixed their dwellings in it : a third 

 revolution has arisen, the sea has letired, and left them dry in 

 that place where we now study them. 



The knowledge of fossil shells, as it at present exists, serves 

 almost exclusively to enable us to recognize different por- 

 tions of the same formation in different places. These re- 

 mains are nearly infallible guides in the examination of a li- 

 mited district, notwithstanding the mineralogical differences 

 cflen presented in the same formation in different parts of it. 

 Let us take, for example, a system of beds, formed as are those 

 in the vicinity of Paris, of alternating beds of limestone, sand, 

 gypsum, and clay. When one of these beds is alone found, 

 it is almost impossible to assign it its true place without having 

 recourse to its zoological characters. How, for instance, can 

 we without these characters distinguish the sands of the plastic 

 clay, from those of the calcaire grossia; and those above the 

 gypsum? 



When, however, the observer compares distant countries 

 and geological basins separated by mountain chains or seas, 

 he does not find the same facility. If, at first sight, he con- 

 siders he has met with species already known, he may also 

 find many others new to him. If he afterwards regards the 

 species, which he at first considered identical, he will remark, 

 after careful examination, a crowd of differences between their 

 forms and those of the species which he had previously collected 



