292 SPECIES OF THE SAME GENUS 



same genus, Cyrena vanikorensis, M. Quoy observes : " Ne 

 ] 'ay ant pas trouvee dans les lieux marecageux, mais sur les 

 bords de la mer, il est probable qu'elle vit a l'emboucliure 

 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 obvious that in 

 such instances the animal must be possessed of the capability 

 of adapting itself to the different characters of the two fluids. 

 This capability exists in much more highly organized ani- 

 mals, such as fishes, many species of which constantly migrate 

 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 depen- 

 dent 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 dolabrifera in company with Neritinae and Melanise. 



The greater number of species of the genus Cerithium are 

 truly marine, chiefly living in sandy bays, like our own Ceri- 

 thium 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 man- 

 grove 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 hyda- 

 tis, found by him in brackish pools on the coast of Chili ; 

 and Mr. Say has described a Bulla fiuviatilis found by Mr. 

 Aaron Stone deeply imbedded in the mud of the river Dela- 

 ware, -j- 



The Littorina?, 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 Paludinse 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 



* Voyage de l'Astvolabe, torn. iii. p. 516. 



f See for this latter instance the Journal of the Academy of Natural 

 Sciences of Philadelphia, vol. ii. p. 179. 



