14 M. D'Omalius D'Halloy on 



the particles are closely united ; and as these diflFerent states 

 of induration are irregularly disposed in the same mass, the 

 hard parts might be said to be owing to the infiltration of a 

 species of calcareous cement.* Above this sandy limestone, 

 are beds of a compact, white, and solid limestone, alternating 

 in the higher parts with greenish marl. I have not dis- 

 covered any shells in place in this formation ; I only found 

 detached fragments of white limestone, containing the inte- 

 rior casts of cyclostoma mumia, and which I believe, from 

 appearances, to belong to tlie inferior beds. A stone is 

 employed for building in these cantons, particularly in the 

 environs of Dormans, containing a great abundance of ceri- 

 thium lapidum. It is a whitish fine grained limestone, a 

 little cavernous, like the freshwater limestone, and which 

 appears to me to come from the beds between the true cal- 

 caire a cerites, and that which contains the cyclostomae ; as 

 it more resembles this last than the common marine lime- 

 stone, I should be inclined to believe that it had also been 

 formed in fresh water, and that these two beds belong to the 

 same system as the clicart of Mantes la-Ville, described page 

 229 of the mineral geography of the environs of Paris. + 



• These solid limestone nodules very frequently occur in the friable 

 strata below the calcaire k cerites, and are seen at Grignon ; at the 

 descent of Beaumont sur Oise ; at Meudon ; at Issy, &c. &c. They 

 project beyond the vertical sides of these beds, and often appear as if pe- 

 netrated by calcareous spar, which gives a radiant appearance to their 

 fracture. — (Note of the Editor of the Annales des Mines.) 



+ I shall here observe that many geological circumstances, joined to 

 zoological characters, make me believe that the cerithium lapidum 

 ought to be arranged with the Potamides of M. Brongniart, or cerithia of 

 the freshwater formations. This shell, differing very little from Pota- 

 mides Lamarkii, appears to me lo possess this peculiarity, that it occurs 

 in the last marine beds, and in the first strata of the freshwater formation, 

 and that it is the only fossil of the marine formation that really occurs 

 in place in the freshwater formation. 



I shall to this add that I have observed a potamides at Etampes which 

 appears to me more slender, and with less defined tubercles than Potamides 

 Lamarkii. I conceive that it may be considered as a separate species, or 

 principal variety, that it may be named P. acurainatus. It occurs in the 



