216 ORGANIC REMAINS. 



depressions, &c. be natural, or only accidental; as the shells must 

 have been liable to suffer great alterations in their form, when passing 

 from the recent to the petrified state. 



The difficulty of distinguishing the genera and species is further 

 increased, by the changes which the very substance of the shells has 

 undergone; their appearance being vaiied, according as they are 

 found in the state of sandstone, limestone, ironstone, pyrites, agate, 

 chalcedony, or calc spar. Shells in the latter state, which admits of 

 many varieties, are generally much thicker than recent shells of the 

 same kind. It would hence appear that, as water expands in being- 

 crystallized into ice, the substance of the shells has expanded in 

 crystallizing into spar. 



On these accounts, we rarely meet with fossil shells exactly 

 corresponding with recent specimens; but if we find a general agree- 

 ment between the one and the other, we may in many cases be 

 satisfied as to their identity. It is a common fault with authors, to 

 multiply the species of fossil shells to an unreasonable degree. Instead 

 of making allowances for the causes of difference now stated, a very 

 trivial variation is with them sufficient to constitute a new species ; 

 though we know well, that there are gi-eat variations in recent shells 

 confessedly of one species. We may add, that allowance should be 

 made for such variations as depend on difference of climate, as well 

 as for those occasioned by the process of petrifying, the difference of 

 the matrix, or of the substance into which the shells has been con- 

 verted ; for an attentive observer may perceive, that at the era or eras 

 of the formation of our strata, the productions of various climates 

 have been blended together, as is the case with the animals and 

 vegetables of the sea and the land. 



We remark further, that though most of the shells be found with 

 the valves adhering, and appear to have been imbedded in the strata 

 while the animal was yet contained between the valves, yet vast 



