308 MR. GRAY ON TESTACEOUS MOLLUSCA. 



the part of the sea in which they are met with may be fresh, like certain parts of the 

 ocean described by Dr. Abel in his voyage to China. It would be highly interesting 

 to procure a verification of this observation. Similar phenomena may not be uncom- 

 mon, for I have myself observed in Torbay a small space in the neighbourhood of 

 Brixham, the water of which was of a different colour and much fresher than that of 

 other parts of the bay. With reference to another species of the same genus, Cyrena 

 VanikorensiSy M. Quoy observes : " Ne I'ayant pas trouvee dans les lieux marecageux, 

 mais sur les bords de la mer, il est probable qu'elle vit a I'embouchure des rivieres 

 qui sont saumatres a maree haute*." 



The third class of cases, in which species of Mollusca that are generally found in 

 the sea are taken in fresh water, is much more rare than the preceding. It is ob- 

 vious that in such instances the animal must be possessed of the capability of adapt- 

 ing itself to the different characters of the two fluids. This capability exists in much 

 more highly organized animals, such as fishes, many species of which constantly mi- 

 grate from the sea and ascend the rivers to deposit their spawn ; but in these cases it 

 is the result of a regular and determinate habit, while in the Mollusca it appears to 

 be entirely dependent on accidental circumstances. 



In some marshes in the Island of Bourbon, in which the water is almost fresh, 

 M. Rang has observed specimens of Aplysia dolahrifera in company with Neritmoe 

 and Melanice. 



The greater number of species of the genus Cerithium are truly marine, chiefly 

 living in sandy bays, like our own Cerithium reticulatum. M. Lesson, however, 

 found C. sulcatum, and Adanson the African species figured by him, in the pools of 

 brackish water, sometimes overflowed by the sea, which are situated between the 

 weeds and the belts of mangrove trees on the shore ; and Mr. Say observes that 

 the small species, called by him Pyrena scalariformis, but which is a true Cerithium, 

 is found in great abundance in the fresh water of Florida Keys. He adds : " it is 

 most certainly a freshwater shell, yet it is destitute of an epidermis." 



The genus Bulla is also truly marine ; but the Rev. Mr. Hennah some time since 

 presented to the British Museum specimens of one of its species, resembling the Bulla 

 Hydatis, found by him in brackish pools on the coast of Chili ; and Mr. Say has de- 

 scribed a Bulla Jluviatilis found by Mr. Aaron Stone deeply imbedded in the mud of 

 the river Delaware-}-. 



The Littorinae, again, are all found either on the sea-shore or in the very brackish 

 water of the mouths of rivers, except two, which although described as Paludince by 

 Pfeiffer and De Ferussac, and formed into a distinct genus by Ziegler under the 

 name of Lithoglyphus, agree with Littorina in every character of shell and operculum, 

 and, as far as I can ascertain from the descriptions, of the animal also. These are 

 the Paludina fusca of Pfeiffer, and the P. naticoides of De Ferussac ; they are 

 truly fluviatile. 



* Voyage de r Astrolabe, torn. iii. p. 516. 



t See for this latter instance the Journal of the Academy of Natural Sciences of Philadelphia, vol. ii. p. 179. 



