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



by experienced eyes, such is by no means the case with the 

 oblique deformations of bivalve shells. In these the pres- 

 sure may not only render one valve more elevated than the 

 other in symmetrical shells, and give them a greater or less 

 resemblance to corbula* or thracia ; but, besides, when it 

 takes place in a vertical direction, passing between the two 

 valves, and which it inclines more or less to the side of the 

 labrum, this oblique deformation may modify the apical angle 

 of a bivalve, and change its form to such a degree, without 

 making it cease to be symmetrical,! that it may become very 

 difficult to distinguish true species from deformations of this 

 nature, which are, however, very common in shells which 

 have preserved their normal position in the midst of argilla- 

 ceous strata !| Not only, therefore, is it necessary frequently 

 to disregard form, but also, in order to distinguish true spe- 

 cies from accidental deformations, it is requisite to commence 

 by seeking other exterior characters, and to compare, in this 

 point of view, all the specimens that have been collected in 

 the same bed and in the same place ; for, in that case, the 

 change of place and of strata must be allowed to have some 

 influence in determining the limits of a fossil species. 



" Referring to what I have already stated respecting the diffi- 

 culties attending the positive determination of fossil species, I 

 would say that these difficulties are so much greater, the moi-e 

 ancient the faiina that we investigate. In fact, the lower a bed 

 lies, the more must the shells it contains have been exposed 

 to dislocations, pressures, and the modifications to fossil forms. 

 If, for example, the determination of species of extreme dif- 

 ficulty in the transition formations, when it is undertaken con- 

 scientiously ; if it is still so in the cretaceous formations, after 

 we enter upon the tei-tiary formations, such as those of the 

 Parisian basin, it ceases altogether, and the determination of 

 the fossil shells of this epoch enters into the category of that 

 of living shells. It is only necessary, for the most part, to 



* This deformation is very fi-equpiit. 



t Tlie Pholadomya are often found in this state, whicli has caused the species 

 to be multiplied without end. 



\ They are found in a great number of placeis in France. 



