267 



of land, &c. M. Brongniart expects to prevent its being 

 concluded that these lymnece, &c. were sea shells, or that 

 revolutions of which, he says, there exists no trace, carried 

 these fresh-water shells into the ocean, and mixed them 

 with the sea shells. 



Brongniart and Cuvier had founded their opinion of the 

 fresh-water origin of the upper beds of gypsum, chiefly on 

 the presence of a shell found in the gypsum, which appeared 

 to be a cyclostoma, and was supposed to be a fresh-water 

 shell. Of these shells two only were found in the gypsum ; 

 one of which, in the possession of Brongniart, had its mouth 

 unluckily concealed ; but the other, in the possession of 

 Faujas St. Fond, as fortunately, had it displayed so as to 

 show that it agreed with cyclostoma mumia of Lamarck, who 

 had only named it so provisionally, since, from its thickness, 

 he had supposed it to be a sea shell. M. Brard hence con- 

 cludes, that the fresh-water origin of the gypsum does not 

 derive any support from this shell.* 



He describes, as existing among the marine fossils of 

 Grignon, two minute land bulimi, the conical and pyramidal ; 

 and one still more minute, the cylindrical, which approaches 

 in form to many of the small species of pupa. This 

 shell has six turns, very projecting, deeply separated, and 

 derives a cylindrical form from the last three turns being 

 nearly of the same diameter. He also mentions impressions 



their having lived in a common medium, contend for the difficulty 

 of distinguishing the difference between these shells ; whilst those 

 who contend for alternations of sea and fresh-water, think the 

 practised conchologist might find but little difficulty : yet Lamarck, 

 after examining and naming sixty species of cerithia, says decidedly, 

 " Les Cerites vivent toutes dans la mer ;" but now another genus is 

 formed of several species of fresh- water shells out of these, showing, 

 certainly, that the distinction cannot be easily made. 



* Annales du Mus. Tom, Ixxii. p. 448. 



