the Environs of Paris. l5 



The green marls are covered by another limestone equal- 

 ly white, but a little less compact than the preceding, pre- 

 senting tubular cavities, characteristic of particular parts of 

 the freshwater formation, and containing a great quantity of 

 shells, among which may be particularly distinguished two 

 species of lymneae and a small araphibulimus,(Bulimus pusil- 

 lus. Brong.) 



Lastly, the whole is surmounted by the millstone without 

 shells, with the sands and clays which usually accompany it, 

 and which cover all the platforms in the vicinity. 



This order of superposition, sufficiently worthy of atten- 

 tion from the series of beds that it exhibits, is still more 

 remarkable for the constancy and uniformity with which it 

 appears in the whole of the country extending from Chateau- 

 Thierry to Reims. 



I regard all the portion of this country placed above the 

 calcaire a cerites, as belonging to the freshwater formations. 



white limestone, forming the irpper part of the freshwater formation ; it 

 is generally transformed into white silex, and often attached to nodules 

 of that substance. 



The true P. Lamarkii occur in the same situation, forming a species of 

 deposit in the midst of a bed, whose upper part consists of a tolerably- 

 dark smoke grey compact limestone, and the lower part of a slightly 

 agglutinated sand, coloured brown by carbonaceous matter. This bed, 

 situated under the white limestone, rests immediately on a thick deposit 

 of sandstone, and sand without shells, and may be compared to the slaty 

 clay bed described in tke work of Messrs. Cuvier and Brongniart, page 

 282. 



I believe that P. Lamarkii, and P. acuminatus have yet been observed 

 only in the second freshwater formation, whilst C. lapidum has not yet 

 been seen beyond the inferior strata of the first freshwater limestone. 



The white limestone of Etampes contains, besides potamides, lymnea, 

 planorbes, and a shell, which has as yet been but very seldom observed ; 

 it is the cyclostoma that M. Brongniart has described under the name of 

 C. elegans antiquum. The numerous individuals that I have seen have 

 appeared to me always thicker, and shorter than those of the recent 

 species ; so that I conceive they ought to be considered as a distinct spe- 

 cies, which may be named C. crassatum. This cyclostoma has yet only 

 been found in the second freshwater formation, whilst the C. mumia is 

 the characteristic fossil of the first formation. 



