MOLLUSC A. 241 



criminated by the petrifactions which they contain. From 

 the difficulty of distinguishing the fossil species, however, 

 joined with our ignorance of their geographical distribution, 

 some mineralogists have not permitted their conclusions to 

 be much influenced by this rule. 



It will likewise be observed, that the shells in the newer 

 strata are but little changed, whilst those in the older rocks 

 are greatly altered in their texture, and in part obliterated. 

 The same power which rendered the rock compact or crys- 

 talline, has likewise exerted its influence on the imbedded 

 remains. In the newest strata, this power has scarcely be- 

 gun to operate ; so that the imbedded shells still retain in 

 perfection their original characters. 



In examining a limestone quarry, for example, the stu- 

 dent will perhaps be surprised to find petrifactions of shells 

 in the bed of limestone, while, in the sandstone covering, 

 he witnesses impressions of plants unaccompanied with 

 shells. In order to gain more correct ideas on this subject, 

 let him repair to a marl bog, and he will there find the bed 

 of marl abounding in shells, while in the bed of sand below, 

 on which it rests, or of peat moss, which covers it, he will 

 find exclusively the remains of vegetables. Here let him 

 study the subject, while the strata are yet recent, and while 

 lapidification is only commencing. There is, however, this 

 difference between the shells in the marl and those in tht 

 limestone, that individuals of the former species still exist, 

 while no living examples of the latter are known. 



GEOGRAPHICAL HISTORY OF FOSSIL SHELLS As the 



geographical distribution of recent shells is a branch of con- 

 chology to which few have devoted their attention, and about 

 which very little is known, we can scarcely expect to find the 

 geographical distribution of the fossil species more fully illus- 



