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



termine, by means of these data, what has been the mode of 

 deposition of the fossils contained in any kind of strata. 



" The shells thus deposited, and more or less covered by 

 posterior deposits, have so remained, with their shell changed 

 into different substances. They have passed into the state 

 of impressions, or else they have reached the condition of 

 counter impression. If, after their deposition, the beds in 

 the state of paste have sunk in their horizontal position, in 

 consequence of the pressure of the whole ;* if they have been 

 dislocated before this pressure, and a sinking or oblique slip- 

 ping of the molecules, in relation to their first horizontal de- 

 position, has taken place, it will be perceived thai all the 

 bodies found in these strata must have been subjected to the 

 same pressure, horizontal or oblique, and will then become 

 deformed in consequence of their relative position. 



" Horizontal pressure, for example, produces a flattening 

 of the shells in the direction of their compression. Accord- 

 ingly, the nauliti, ammonites, in all the parts which were con- 

 vex, are more or less flattened, and often become as thin as 

 a sheet of paper.f Bivalves placed on the side lose half of 

 their thickness, or become quite flat, and without convexity. J 

 We may likewise observe this simple compression in shells 

 natui'ally compressed ; but when it takes place in conical 

 shells it may be conceived to change the specific characters 

 altogether. § 



" If deformation, in the direction of the compression of shells, 

 may change their shape, this deformation will be much more 

 considerable when it is exerted in the direction of their 

 length. This takes place principally when the gasteropodes 

 and acephales have preserved their natural position. Indeed, 

 conical shells will become entirely flat,|| or their spire will 

 change altogether from a spiral angle, and from being ele- 



* This has talu'n place in all the formations. 



t This is seen in many amnioni'tes of the foliated lias. 



X The possidonies of the lias present tliis depression. 



§ This deformation takes place in the Trochi and Pleurotomarise. 



II The Patella and Orblcula. 



