66 M. Alcide d'Orbigny on Living and Fossil Molluscs. 



tially changed in their nature. Such a shell composed, for 

 example, of molecules of carbonate and phosphate of lime, 

 and horny or mucous animal molecules, sometimes still re- 

 tains, in its composition, some of the carbonate of lime ; but 

 then this substance, at least if it be not of a lamellar texture, 

 as in the shells of certain genera,* does not preserve its 

 original appearance. The mineral matter by which it is 

 replaced, is formed of carbonate oflime,t silica,^: sulphuret of 

 iron,§ hydrated iron, || oligistic iron,1[ sulphate of strontian,** 

 sulphate of barytes,tt lead,^ or any other substance, no 

 longer possesses its primitive internal texture. It is the 

 mineral matter in its ordinary aspect which occupies the place 

 of the shell. When shells have only changed tlieir nature, 

 they preserve all their characters, and it is easy to study 

 them. 



" Shells enveloped in particles of clay, marl, or lime, after 

 their deposition in ancient seas, which have been afterwards 

 entirely destroyed by the action of chemical agents, and left 

 their places empty, present greater difficulties. When the 

 void has remained untouched, it shews on the one side the 

 impiession of the exterior characters, and on the other that 

 of the internal characters. It is then for the observer to en- 

 deavour to reconstruct, by artificial means, or to recognise 

 the character of the genera and species of the shell by the 

 union of the two impressions remaining in the rock. A single 

 valve of the acephales presenting at once the exterior form 



* Ostrea and Terehmlulo . 



t Such are found in Prance at an infinite number of places. 



I All the shells of Uchaux (VaucluseJ and Launoy (Ardennes), contained in 

 the cretaceous and Oxford strata, are in this state. 



§ The greater part of the shells at Vaohes-Noires (Calvados) are thus trans- 

 formed. 



II This transformation is very common. 



^ This is met with in the vicinity of Seraur (Cote d'Or), in the lias. 



** I have collected some of. these in the Neocomian stage, near St Dizier 

 (Hautemarne). 



tt I possess belemnites in strontian, discovered by M. Delanoue, in the lias 

 of the neighbourhood of Nontron (Dordogre). 



IJ I have some gryphees thus transformed, from the neighbourhood of 

 Seuiur. 



