M. A. d^Orbigny on the Distribution of Littoral Mollusca. 43 



spective faunas which inhabit- them, led him to the following re- 

 sults : — In ninety-five genera cited, fifty, or much more than half, 

 are found only on one side, whilst forty-five only are common to 

 the two seas. From this he concludes, that the configuration of 

 the two coasts of South America, the one abrupt on the side of 

 the Pacific, the other rising in a gentle acclivity from the Atlan- 

 tic, have a greater influence upon the whole than the parallelism 

 of the zones of latitude which the local faunas of the two oceans 

 traverse equally. 



In a fourth chapter, devoted to general deductions and con- 

 clusions, the author considers separately the action of the cur- 

 rents, the temperature, and the orographic configuration. 



The general currents tend, by their incessant action, to diffuse 

 upon all the points where they pass, the mollusca which can bear 

 a great difi'erence of temperature. In fact, in the Atlantic twelve 

 species extend over nineteen degrees, and in the Pacific fifteen 

 species are distributed over twenty-two degrees of latitude, tra- 

 versing several difierent zones of heat, and cease to exist at the 

 furthest northern limits of the currents, as is seen at Brazil and 

 to the north of Callao (Peru) . Thus we must, without any doubt, 

 attribute to the general currents that influence of unequal value 

 which carries the littoral mollusca of the cold regions in the At- 

 lantic as far as the tropic only, and in the Pacific as far as eleven 

 degrees more to the north. 



The author finds the currents to have two opposite influences : 

 by their continual action they tend evidently to diffuse the litto- 

 ral mollusca beyond their natural limits of latitude ; but when 

 they are distant from the continent, as at the Falklands, when 

 they double a cape advanced toward the pole, as at Cape Horn,- 

 or when they abruptly leave the coasts, under the hot regions, as 

 at Payta, they then serve to isolate local faunas. 



The effbct of temperature is to confine species within more or 

 less restricted limits ; the proof of which lies in the number of 

 mollusca peculiar to the different zones of heat traversed by the 

 general currents, and above all in the sudden difference which is 

 remarked between the composition of the local faunas of Payta 

 and that of the parts situated to the north of Rio Janeiro. In 

 fact, as soon as the action of the currents ceases to be felt, the 

 temperature at once resumes all its influence, and a fauna pecu- 

 liar to the hot reg-ions begins to appear. 



The orographic configuration of the coasts is marked by the 

 different zoological forms which are observed between the two 

 oceans : in fact, independent of the numerical amount of the ge- 

 nera which have been spoken of, it is easy to convince ourselves 

 that the genera which predominate in the Pacific live principally 

 on the rocks, whilst those of the Atlantic, which are wanting on 



