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PROCEEDINGS OF THE 



[April, 1857. 



Geographical Relations. 



In considering this subject, two sets of relations present them- 

 selves between different Faunas, and these may be termed, 1st, 

 continental relations; 2d, isothermal, or climatic relations. The 

 first are those typical relations which, existing between all the 

 genera composing the Fauna of a continent, impart to that Fauna 

 an idiosyncrasy distinguishing it from the Faunse of other conti- 

 nents. The second are those analogical relations found generally 

 between the specific forms, more rarely between the groups of 

 higher value, which belong to the same isothermal zones, but to 

 different continents. Lastly, there are certain relations which are 

 established by comparatively accidental circumstances, such as 

 the course of oceanic currents, &c. All of these relations appear 

 to find exemplification in the Fauna of Charleston Harbor, so far 

 as regards the Hydroid Medusae. 



There are but two harbors in America whose Medusas have yet 

 been made known, namely, those of Boston and Charleston. In 

 the former, from its northern latitude, we should naturally expect 

 to find a less variety of genera and species than in the latter. 

 Yet it is probable that Prof. Agassiz' forthcoming work will give 

 us a knowledge of a greater number of forms than were described 

 in his admirable memoir before quoted, devoted as it was more 

 especially to an investigation of the structure of certain species, 

 than to a combined view of all the genera comprised in the Fauna 

 of Boston Harbor. Nevertheless, we see that the genus Hippo- 

 crene, as distinguished from the European Bougainvillea, is com- 

 mon to both Boston and Charleston Harbor. The Fauna of Grand 

 Manan, as made known by Stimpson in his valuable paper, pub- 

 lished in the Smithsonian Contributions to Knowledge, does not dif- 

 fer from that of Boston, except in the presence of the genus Acaulis, 

 which has not yet been noticed from any other locality. The 

 Fauna, of Long Island Sound, also, is partly known through the 

 joint contributions of Ayres, Agassiz, and Leidy, and if it is not 

 premature to form conjectures from only partial knowledge, we 

 may consider it different from the Fauna of the eastern shore of 

 New England.* In it we find the genus Nemopsis, which has 

 also a representative in Charleston Harbor, but has not yet been 



* The genus Clava, however, not yet observed in Charleston, is found on both 

 shores of New England. 



