Institute of France. 53 



thfeir stratification, and the organised bodies of which they con- 

 tain the leniains. 



The stonv beds which contain fresh-water shells only, so 

 manv of which Messrs, Cuvier and Brongniart have discovered 

 in the environs of Paris, and which Messrs. Brongniart, Onia- 

 lius de Hallov, Marcel de Serres, Daudebart de Ferussac, &c. 

 have recognised in other countries, have particvdarly excited the 

 attention of naturalists, who have set about distii)gui-,hing the 

 sliells of fresh from those of salt water. Messrs. Marcel de 

 Serres and Fcrussac have each given in a memoir upon this 

 subject. The species alone, according to the former, may be 

 offered in proof, and not the genera; for most of the genera have 

 both fluviatile and marine species : the varieties even are not 

 unworthy of notice ; for the same species, according to the ob- 

 servations of tlie author, sometimes changes its form, so as to 

 deceive those who have not observed the different transitions ; 

 and the diificiilty increases when it is required to classify shells 

 in the fossil state, wlien the epidermis, the hairs, and all the otiier 

 fugitive characters have disappeared. 



There are species, particularly among the operculcfe, which 

 live both in salt and fresh Avater, and which are consequently 

 more abundant at the mouths of rivers : and we may observe 

 among fossils traces of this habitude; for our fresh-water beds 

 contain in certain places a species of the potamidus, a genus 

 whicli is also met with at the mouths of rivers. 



M. Marcel de Serres visited on purpose the salt-water pits on 

 the shores of the Mediterranean, in order to examine the shells 

 which tiiev contain : he there observed paludines similar to those 

 which form extensive beds in the environs of Mentz, where se- 

 veral sea shells are also found. A geologist who had con- 

 founded these paludines with one of the hulimi of our fresh- 

 water fctrata, concluded that the latter are sea shells as well as 

 tlie others ; but dc Serres clears up this mistake, and shows tliat 

 t'ley are not only species, but different genera. 



This observer has examined the limits of these migrations of 

 animals and plants from salt to fresh water, and vice versa. 

 He ascertaine i that no animal and no plant can resist a brine 

 of eight degrees : he has distinguished, both among animals and 

 plants, the species which adliere to the shores of the sea on ac- 

 count of the sand, and whicli can exist in other sandy places 

 also ; those which are attracted by the salt alone, and which 

 exist very vvell in salt pits or lakes at a distance from the sea; 

 and, fm^dly, those which require the sea as it is, and do not re- 

 move from it. 



These observations prove that it is not always easy to decide 

 D3 if 



