M. Alcide d'Orbigny on Living and Fossil Molluscs. 67 



and the hinges, may admit of pretty easy determination ; but it 

 is not always so with the gasteropods, and particularly the 

 bivalves, when they have been shut, and left only what has 

 been improperly called the kernel or interior mould, which I 

 would designate as the internal impression ; for then a great 

 number of conchyl logical characters, such as those of the 

 hinges, have often disappeared, and in many cases it is ex- 

 tremely difficult to determine the genera and species. But 

 if the difficulties begin with the internal impressions of en- 

 tire bivalves pretty well preserved, they increase when the 

 state of preservation becomes still less complete. 1 refer to 

 counter-impressions, when, for example, the shell has com- 

 pletely disappeared, in an argillaceous or calcareous bed in 

 a still unsolidified state ; and when the impression produced 

 by the weight of the superior beds, tends to make the bed 

 more compact by bringing all the parts towards each other ; 

 then the void left in place of the shell disappears, and the 

 interior and exterior impression, united and brought in con- 

 tact, sometimes completely attenuate the internal characters, 

 or at least produce an appearance which is neither an inter- 

 nal or external impression, but rather a combination of both. 

 In these circumstances, of very frequent occurrence, the cha- 

 racters are altered, and very difficult to recognise.* It is 

 commonly not till after having handled and seen thousands 

 of fossil shells of this nature, that we can succeed in per- 

 ceiving, in the most fugitive characters, what must have ex- 

 isted in the primitive state. 



" A second cause of error is owing to the disappearance 

 from certain strata of the substance of shells, and tlie pre- 

 servation of certain otliers in the same subjects. This mo- 

 dification, very common in the ancient formations, t is like- 

 wise so in the most modern.;}: We observe, for example, the 

 exterior layer of the shell disappear, and along with it the 

 specific characters, leaving a second, which is, for instance, 



* Almost all the fossils of the sujjerior Oxford stage in the \ Iciiiity of La 

 liochelle are in this condition. 

 t This is seen in the Produclut. 

 \ In llie cretacious fossils ol .Maus fSarthe), alteration is frequent. 



