60 M. Alcide d'Oi-bigny on Living and Fossil Molluscs. 



regnlate the geographical distribution of coast molluscs, may 

 be reduced to two contrary actions, — currents which have the 

 tendency to spread, wherever they pass, the species indiffe- 

 rent to temperature ; currents, temperature, and orogra- 

 phical configuration, which tend, on the contrary, to restrict 

 and localise beings within limits more or less extensive. 



" I can further deduce from my researches the following 

 conclusions, which are interesting in a palseontological point 

 of view : — 



" Two neighbouring seas, communicating with each other, 

 but separated only by a cape advancing in the direction of 

 the pole, may have their faunas distinct. 



" Distinct faunas may exist at the same time, by the sole 

 action of temperature, in the same ocean and on the same 

 continent, according to the different zones of temperature. 



" Under the same zone of temperature, and on the neigh- 

 bouring coasts of the same continent, cun'ents may deter- 

 mine the particular faunas. 



" A distinct fauna from the fauna of the nearest continent 

 may exist in an archipelago, when the currents have the 

 effect of insulating it. 



" Distinct faunas, or at least very different from each other, 

 may appear on neighbouring coasts, in consequence of the 

 sole influence of orographical configuration. 



'• When we find the same species over an immense extent 

 of latitude, in the same basin, currents must be regarded as 

 the cause of it. 



" Identical species, iu two neighbouring basins, indicate 

 direct communication between them. 



" The largest tributaries do not absolutely exercise any in- 

 fluence on the composition of the marine faunas of sea-coasts." 



These researches on geographical distribution have afford- 

 ed M. d'Orbigny most valuable data for studying the geolo- 

 gical distribution of these same animals. 



" After having given a brief view of my investigations re- 

 lative to the geographical distribution of living molluscs, I 

 ought to say something of the distribution of the species 

 buried in the strata which compose the crust of the earth. 



