240 



FOSSIL CONCHOLOGY. 



In the tertiary beds of this group, nearly fourteen hundred species 

 of shells have been found, of which thirty-eight species are analogous 

 to existing species, or about three in every hundred. Only forty-two 

 of these species appear in the upper tertiary, and none of the four- 

 teen hundred species found in this group, have any analogy with those 

 found in the secondary strata, not even in the most recent or chalk 

 formation. 



The second or middle group comprises the marls of Touraine, and 

 other parts of the Loire, a great part of the basin of the Gironde, 

 of Dux of Austria, Hungary and Poland, and a small portion of the 

 sub-Appennine hills, in the environs of Turin. Geologists and natu- 

 ralists had before only admitted one group of tertiary strata in Aus- 

 tria and Italy. 



Of nine hundred species of fossil shells found in this group, and 

 compared by M. Deshayes, one hundred and sixty are analogous to 

 living species, or eighteen in every hundred, and one hundred and 

 thirty species have continued to live, during the formation of the up- 

 per or more recent group. 



The upper group comprises the sub-Appennine hills, the tertiary 

 strata of Sicily, those of the Morea, the small basin of Perpignan, 

 and the small basins bordering the Mediterranean. In this group M. 

 Deshayes is inclined to place the Norfolk crag, at least until its char- 

 acters shall be better known. 



M. Deshayes has recognized seven hundred species in the upper 

 group, of which the greater half are analogous to living species. 

 Thirteen species alone, M. Deshayes observes, have yet been found 

 common in all the three tertiary groups, and have resisted the destruc- 

 tive causes that have successively modified the organization of subma- 

 rine animals. The living species, analogous to the fossil shells in the 

 more ancient and middle groups, are chiefly inhabitants of tropical 

 climates, whereas the greater number of species, found in the most 

 recent group are analogous to those now living in European seas. 



The results of M. Deshayes' researches, if fully confirmed, would 

 establish the following rules for determining the relative ages of strata. 



1. That in proportion to the greater number of fossil species in 



strata analogous to living species, such strata may be deter- 

 mined to be more recent. 



2. That a great change in the organization of fossil species, and 



in the proportion of the number analogous to living species, 

 ought to be considered sufficient to constitute different for- , 

 mations. 



3. That the different tertiary basins, were not formed or filled 



contemporaneously. 



Before admitting the conclusions of M. Deshayes, it will be right 

 to pause, and consider well how httle we know of the inhabitants of 

 the shells which are divided by conchologists into such a multitude 



