294 SPECIES OF THE SAME GENUS 



that oysters can exist in water absolutely fresh ; for in the 

 Museum of the Bristol Institution there is a large grovip 

 said to have been dredged up in a river on the coast of 

 Africa where the stream was so sweet as to have been used 

 to water the ship. To these shells are attached specimens 

 of Cerithium armatum ; and the person by whom they were 

 presented to the collection stated that Cardium ringens was 

 found abundantly in the same situation. 



The genus Cucullasa, again, is universally considered as 

 truly marine ; but Mr. Benson has found in the Ganges a 

 small shell belonging to it, regarded by him as an Area, but, 

 on account of its freshwater origin, formed into a new 

 genus under the name of Scaphula. 



On this subject I may observe, that I was some time ago 

 informed that Area senilis was found in the rivers of Africa 

 in company with Galatea radiata : M. Cailliaud, 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 dis- 

 tance 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 con- 

 diment 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, obtained in such 

 abundance from the West Indian Islands. Nilsson too, as 

 before mentioned, has noticed the Neritina fluviatilis, 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 Lym- 

 nasa balthica and L. succinea ; and M. Rang found Neritina 

 auriculata in similar situations. 



According to the observations of Olivier, the Ampul - 

 laria ovata inhabits Lake Mareotis, where it is taken in 

 company with marine shells found also in the Mediterra- 

 nean ; and I have lately received (dead) specimens from 



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

 p. 258. 



