454 FOSSIL INVERTEBRATED ANIMALS. 



the genera which are found in the hving state, and also in the 

 fossil, we shall ascertain that the most ancient contain sixty^ 

 five, those of the chalk forty-two, and the most recent, one 

 hundred and seventy-two. A different proportion obtains re- 

 specting the bodies which are found in the fossil state alone ; 

 the most ancient strata contain sixty-three genera, the chalk 

 contains one-and-thirty, and the most recent strata thirty only. 



It Pias been said, that ammonites were found in the London 

 clay, but this report does not seem to rest on very sufficient 

 authority. 



M. de Humboldt, in his work on the Independence of 

 Formations, has stated, that among the fossil shells the uni- 

 valves predominate, as they do at this day, in the living state, 

 under the tropics. The following is the result presented by a 

 tabular view of them : — The number of the genera of uni- 

 valves exceeds that of bivalves by eleven for those of the 

 living state only ; twenty-four for those found equally in the 

 living and the fossil state, and five for those in the fossil state 

 exclusively. It is less by sixteen for those found in the strata 

 anterior to the chalk, and by nine in twenty-five for those found 

 in that substance : but it becomes greater again in those found 

 in the most recent strata, for there the genera of bivalves rise 

 only to fifty, while the univalves amount to eighty-nine. 



With respect to species, the number of univalves in the 

 living state exceeds that of bivalves by eight hundred and fifty- 

 eight ; and in the fossil state, the number of univalves exceeds 

 that of bivalves by four hundred and thirty- three. 



It was supposed that there had been remarked, at Orglandes 

 and Hauteville, in the department of the Manche, a bed of 

 coarse or crag-limestone, analogous to that of Grignon, and 

 situated under a stratum of chalk foundation. But this could 

 not have taken place without some catastrophe, which dis- 

 placed the strata, or, perhaps, the beds of chalk formation 

 having left empty spaces between each other, into which the 

 composition of the lower stratum might have introduced itself. 



