1821.] On Freshwater and Marine Shells. 309 



Article XIV. 



On the Means of distinguishing between Freshwater and Marine 

 Shells, independently of the Animal Inhabitant. By G. B. 

 Sowerby, FLS. 



(To the Editor of the Annals of Philosophy.) 



SIR, 

 I have been lately under the necessity of asserting the fresh- 

 water nature of many of the shells found in a fossil state in a 

 stratum, which has been described by Mr. Webster (I believe, 

 the only person who has spoken decidedly about it) as wholly 

 marine. 1 am not aware of how much consequence the question 

 may be deemed in a geological point of view ; but it does appear 

 to be of some consequence that facts should be viewed as they 

 really exist, and absolutely necessary that they should be so 

 viewed before we can hope to arrive at certainty in our conclu- 

 sions. I may, therefore, perform an acceptable service to science 

 in general, and to Geological Science in particular, if I endeavour 

 to point out some of the means by which these two Classes of 

 Freshwater and Marine shells may be distinguished from each 

 other. The subject is important fforit is by knowing the nature 

 of the fossil shells contained in a stratum that we are enabled to 

 decide upon the nature of the stratum itself; and it is also diffi- 

 cult : for if we dismiss the animal inhabitant and the Generic 

 Characters of the shells themselves, there will remain scarcely 

 any general character, or circumstance, upon which to decide ; 

 but it will be recollected that the student in Geology is always 

 under the necessity of deciding without the animal, and, fre- 

 quently, he must decide without having enough of the Generic 

 Characters. It may be said, that having ascertained the nature 

 of some of the shells contained in a stratum, he may conclude by 

 a sort of natural induction, that the remaining species belong to 

 the same Class ; but if he attempt to do this, he will soon rind 

 himself in error, for I have lately shown that there exists a bed 

 in which shells decidedly freshwater are mixed with others as 

 decidedly marine ; and De Ferussac appears to have made a 

 similar observation, if I understand rightly the following passage 

 from a letter received lately from him : " Les echantillons que je 

 connois me font penser qu'il existe une couche de melange, ou 

 vos Cyclades, les Melanopsides et des Paludines se trouvent 

 avec des Coquilles marines, entr'autres des Cerites et des Huitres." 

 Some have thought the comparative thickness of the shelly 

 matter might serve as a general distinguishing mark. (Webster 

 in Transac. of Geolog. Soc. vol. ii. p. 211, where, speaking of the 

 " Lower Freshwater Formation," he says, " On a careful exami- 

 nation, I was not able to discover any mixture of marine shells 



