450 FOSSIL INVERTEBRATED ANIMALS. 



monites, and other shells which are never seen in the upper 

 chalk of the environs of Paris. But it may be remarked that 

 the bivalve shells are there in a much greater proportion than 

 the others. 



The silex, which is found in the chalk, has seized upon shells 

 and other marine productions, and the echinodse, in particular, 

 are often filled with it ; but it is very remarkable that it has 

 only seized on bodies belonging to the class of those which 

 are usually preserved. 



Some authors tell us that the silex found in shells has 

 been formed by the animal matter which they contained ; but 

 it is easy to be convinced that it could not have had such an 

 origin in the echinodse which are filled with it, and the exterior 

 of which is covered with oysters, with cranise, and with other 

 bodies, which could not have lived there but after the death of 

 the animal on whose testa they are found. If these examples 

 are not deemed sufficient, many others may be adduced, which 

 prove that the siliceous moulds have been formed in the 

 ananchites after the interior of the testa had been invested 

 by crystals of calcareous spath. 



The siliceous moulds of galerites, and other echinodse, which 

 are found at the surface of the earth without testa, have very 

 probably been covered by silex while they remained in the chalk 

 strata from which they have been derived. The testa was 

 probably destroyed when the strata were washed and carried 

 off by the rains, and the shells thus exposed to the injuries of 

 the weather and accidental shocks ; for, in the chalk strata, no 

 siliceous moulds are found without being accompanied by the 

 testa in which they were formed. 



By rubbing with a brush a piece of the chalk found at 

 Meudon, in France, after having moistened it, it appears 

 sonorous when it is dried ; and it is observed to be traversed in 

 all directions by veins, like the marbles of which we have 

 already spoken ; but no shells in it appear to have been traversed 

 by those veins, as we have seen to be the case in the marbles. 



