96 THEORY OF THE EARTH. 



The Ichthyosauri, the Plesiosauri, several spe- 

 cies of Tortoise, and several species of Crocodile, 

 are found beneath the chalk, in the deposits com- 

 monly called Jura formations. The Monitors 

 of Thuringia would he still older, if, according to 

 the W^ernerian School, the copper-slate in which 

 they are contained, along with a great variety of 

 fishes supposed to have belonged to fresh-water, 

 is to be placed among the oldest beds of the se- 

 condary formations. The enormous crocodiles 

 and the great tortoises of Maestricht, are found 

 in the chalk formation itself; but these are ma- 

 rine animals. 



This earliest appearance of fossil bones seems, 

 therefore, already to indicate, that dry lands and 

 fresh waters had existed before the formation of 

 the chalk deposits. But neither at this period, 

 nor while the chalk was forming, nor even long 

 after, have any bones of land-mammifera been 

 encrusted ; or, at least, the small number of these, 

 which are alleged to have been found in strata of 

 these dates, forms but a trifling exception. 



We begin to find bones of marine mammifera, 

 namely, of lamantins and seals, in the coarse 

 shelly limestone which covers the chalk in the 

 neighbourhood of Paris ; but there are still no 

 bones of terrestrial mammifera. 



Notwithstanding the most assiduous investiga- 

 tion, I have not been able to discover any distinct 



