44 M. A. d'Orbigny on the Distribution of Littoral Mollusca. 



the southern side, inhabit only the sandy bottoms. It is seen 

 that the difference of orographic configiu'ation of the coasts of the 

 two oceans which wash South America exercises, by these condi- 

 tions of existence more or less favourable which it offers to the 

 littoral mollusca according to their genera, an immense influence 

 upon the zoological composition of the faunas which inhabit 

 them. 



The author states it as a negative fact, that the greatest aflflu- 

 ents, the Plata for example, which at its mouth is 128 kilometres 

 wide, have absolutely no intluence upon the composition of the 

 marine faunas of their environs, 



M. d'Orbignj^ deduces from the facts observed by him the fol- 

 lowing conclusions, which apply immediately to the palseontologic 

 faunas of the tertiary deposits : — 



1. The faunas of two neighbom-ing seas, having an intercom- 

 munication, but separated only by a cape advanced toward the 

 pole, may be distinct. 



2. There may exist, at the same time, by the sole action of the 

 temperature, in the same ocean and on the same continent, di- 

 stinct faunas, according to the different zones of temperature. 



3. Under the same zone of temperature, upon coasts in the 

 neighbourhood of one and the same cm-rent, the currents may 

 determine particular faunas. 



4. A fauna distinct from the fauna of the nearest continent 

 may exist upon an archipelago when the currents isolate it. 



5. Distinct faunas, or at least differing much among them- 

 selves, may exist upon neighbouring coasts, by the sole action of 

 orographic configuration. 



6. ^'\Tien the same species are found over an immense extent 

 of latitude, in the same basin, the currents will be the cause 

 of it. 



7. The identical species between two adjoining basins indicate 

 du'ect commimications between them. 



8. The greatest affluents have absolutely no influence upon 

 the composition of the neighbom-ing marine faunas ; thus all the 

 deductions which have been dra\^'n from them, in the case of the 

 tertiary basins, become illusory. 



The author concludes by a final palfcontological comparison. 

 He has said that, \\-ith the exception of one species common to 

 the two American oceans, all the others were, in the actual fauna, 

 peculiar either to the Atlantic or to the Pacific, and the ensemble 

 of the genera was very different in the two seas. The compari- 

 son of these results with the deductions di-awm from the totality 

 of the fossil shells of the lowest tertiary beds of South America, 

 proves that these last, although differing specifically, are never- 

 theless in the same geographical conditions as the actual fauna. 



