INHABIT DIFFERENT SITUATIONS. 295 



the locality indicated. The same species was found by 

 M. Cailliaud in freshwater lakes in the Oasis of Siwah, 

 where it is called Bozue, and eaten as food. It thus ap- 

 pears to be found both in fresh and brackish water. Two of 

 the species referred to this genus by Lamarck, his Ampul- 

 laria avellana and A. fragilis, are truly marine ; but they 

 differ from the others in animal and operculum, as well 

 as in the sinuated form of the outer lip of their shell. 



The common cockle of the shops, Cardium edule, is 

 constantly to be seen in the ditches of brackish water in 

 the neighbourhood of Tilbury Fort, which gradually be- 

 come more or less fresh in proportion to the quantity of 

 rain that falls between the periods of opening the sluices. 

 It is to be observed that the specimens found in this 

 situation are rather thinner and more produced posteriorly 

 than those usually found in the sea. The species in ques- 

 tion is also, according to Nilsson, found in the brackish 

 water on the shores of the Baltic, bx;t I am not aware 

 whether or not it is there subject to a similar variation in 

 form. Nilsson observes, however, that the marine species 

 found in those localities are generally smaller than those 

 found in other situations. 



From this list of exceptions to the general rules which 

 have commonly been regarded as decisive of the localities 

 inhabited by recent shells, and of the nature of the de- 

 posits in which the fossil species are found, it is manifest 

 that those rules cannot safely be made use of for practi- 

 cal purposes without considerable reservation. 



J. E. Gray. 



