M. Alcide d'Orbigny on Living and Fossil Molluscs. 71 



vated, will become depressed, or even horizontal.* Accord- 

 ingly, we must not take into account the spii'al angle of the 

 shells of the gasteropods in the state of counter-impressions 

 deposited in calcareous and argillaceous beds, until we have 

 compared them with a great number of individuals not de- 

 formed. 



" In regard to the acephals, deformation is one of the great 

 causes of error. Such a shell being naturally oblong, when 

 shortened on itself by vertical pressure, may become of 

 greater breadth than height,! and undergo such a change in 

 appearance as to be readily transferred from one genus to 

 another. When, on the contrary, this pressure is exerted in 

 the ti-ansverse direction of a shell, that is to say, from the 

 hooks on the edge of the paleum, such a species, though at 

 first round, may become oblong or even elongated, + under- 

 going a complete modification. 



" The deformation produced by an oblique pressure, regard 

 being had to the compression, to the length or the breadth of 

 the shells, is more easy to determine in certain cases ; but it 

 is, on the contrary, the most difl&cult of all to establish in 

 certain others. Oblique pressure has produced in the ce- 

 phalopods, and in bellerophon, those elliptical spires which 

 have given rise to separate genera. § Some authors have 

 likewise supposed that they have detected in it a distinctive 

 specific character. H This same deformation likewise renders 

 the spiral convolution elliptical in the gasteropods, by throw- 

 ing the summit laterally, sometimes on one side, sometimes 

 on the other.! If these deformations are easily understood 



* I have noticed this deformation in many kinds of Trochus and Pleuroto- 

 mai'ia. 



t I possess the same species in all these deformations, which shall be figured 

 at the head of each class. They are the Cardium hillanum and a I'anopcBa from 

 La Malle (var). 



I We find these defonuations principally in the strata near raoautains, as at 

 Grasse (var.), at C'astellane (Lower Alps), in the Corbieros (Ande), and in a 

 multitude of other places where the strata have been dislocated. 



§ The genus Ellipsolites of Montford, originally adopted, since rejected by 

 Sowerby. 



II The BelkrophoH obliquui of MM. I'otiez and Micliaud, is only a deformation 

 of this kind in D. Munsterii. 



% This is exemplified in some I'leurotomaria; in my possession. 



