310 MR. GRAY ON TESTACEOUS MOLLUSCA. 



however, assures me that this is by no means the case, the shells in question being 

 found near the mouths of the rivers, but never in the rivers themselves. 



One of the most decisive facts regarding the finding of the same species of shell in 

 both salt and fresh water is noticed by Say*. Speaking of Theodoxus reclinatus, he 

 observes, " I found this species in great plenty, inhabiting St. John's river in East 

 Florida, from its mouth to Fort Picolata, a distance of one hundred miles, where the 

 water is potable. It seemed to exist equally well where the water was as salt as that 

 of the ocean, and where the intermixture of that condiment could not be detected 

 by the taste." The shell in question is determined, by specimens which I received 

 from my late friend himself, (to whom science is so deeply indebted, and especially 

 for his researches into the zoology of North America,) to be the Neritina Meleagris, 

 pbtained in such abundance from the West Indian Islands. Nilsson too, as before 

 mentioned, has noticed the Neritina Jiuviatilis, which in this country is not observed 

 to inhabit ditches in the neighbourhood even of brackish water, living on the coasts 

 of the Baltic, in brackish situations, in company with Lymncea Balthica and L. suc- 

 cinea ; and M. Rang found Neritina auriculata in similar situations. 



According to the observations of Olivier, the Ampullaria ovata inhabits Lake Ma- 

 reotis, where it is taken in company with marine shells found also in the Mediterra- 

 nean ; and I have lately received (dead) specimens from 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 appears to be found both in fresh 

 and brackish water. Two of the species referred to this genus by Lamarck, his Am- 

 pullaria 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 question is also, according to Nilsson, found in the brackish 

 water on the shores of the Baltic, but I am not aware whether or not it is there sub- 

 ject 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 re- 

 garded as decisive of the localities inhabited by recent shells, and of the nature of the 

 deposits in which the fossil species are found, it is manifest that those rules cannot 

 safely be made use of for practical purposes without considerable reservation. 

 * Journal of the Academy of Natural Sciences of Philadelphia, vol. ii. p. 258. 



