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



smooth, while the former was striated,* or striated while the 

 first was smooth. I It follows from this, that in many cases, we 

 cannot come to positive conclusions, without bringing together 

 a greater number of specimens. This is likewise the case 

 with states of fossilisation, in which points and tubercles are 

 replaced by depressions.^ long points by small drops, &c.§ 

 One of the most remarkable modifications is that in which 

 the external layers of a shell are always preserved in the 

 rock, while the internal fibi'ous layers almost always dis- 

 appear. II We may thus easily mistake the impression of the 

 internal parts destroyed, for bodies altogether diflFerentl 

 from the first. 



" A third cause of error, against which it is necessary to 

 guard, is the state of preservation in which shells were be- 

 fore they became fossil. Every one may perceive, from ex- 

 amining the edges, that shells separated from their animal, 

 are exposed to numerous causes of destruction. The least 

 that can happen is, that they become rubbed, being rolled 

 along by the motion of the water. Supposing that the same 

 things took place before our epoch that occur in the pre- 

 sent time, we must believe that shells exposed on shores to 

 the incessant action of the waves, would necessarily become 

 rubbed. We find, in point of fact, many beds in which the 

 shells are rolled ;** and as they may render striated shells 

 smooth, attenuate or change all the characters, it must be 

 taken into account in modifications of this kind." 



The deformation of fossil shells is likewise an important 

 fact, and may often cause errors in the specific determina- 

 tions. 



" Although these deformations are of different importance, 



* This is seen in Cardium. 



t The Petunculus especially, and Area, exhibit this character. 



J I have seen thi«! particularly in Cardium productum brought from Uchaux 

 (Vaucluse). 



§ This modification is common in the same species. 



II This takes place in the liippurites and Itadiolites. 



^ Witness M. Defranca's genus lodamia. 



** This is seen in the inferior sandstones of the Turonian stage at Maus 

 (SJarllie), in the Coral-rag of St Mihiel (Meuse) at Tonnerre (Yonne), &c. 



