FOSSIL INVERTEBRATED ANIMALS. 443 



testa has been changed into silex. That of certain shells, found 

 in the green sand of Blackdown, are in like manner changed 

 into this substance. Shells are often observed which have been 

 seized by the silex, or internal moulds, wherever formed of it ; 

 but it may be remarked, that the testa is but rarely changed 

 into this substance. 



We may believe that the matter which forms the siphon of 

 the ammonites is not exactly the same as that of the rest of 

 the shell ; for it has sometimes resisted, while the other parts 

 have been dissolved. 



In certain localities, both in France and England, in the 

 strata of the lower chalk, ammonites are found, the testa of 

 which, in the last turbination, after having been filled with the 

 matter which composes the stratum, seems to have disappeared 

 — while that of the partitions, the siphon, and all that was in- 

 ternal, have been preserved ; so that, in those particular parts, 

 these shells are seen with their testa, such as they were when 

 abandoned by the mollusca which formed them. There is 

 reason to believe that the waters in which these shells, as well 

 as the baculites, existed, and with which they were filled, did 

 not contain substances proper for the formation of crystals — 

 as in the more ancient depositions of Nevers, Caen, and others. 

 Doubtless, the absence of these substances is the cause why the 

 chalk strata are not found petrified, like the last mentioned. 



Respecting the ammonites whose testa has disappeared, there 

 remains only the internal mould ; and the mould of the exterior, 

 in concave, and very generally all the turbinations, have been 

 soldered together after the disappearance of the testa. It was 

 also after this disappearance that the vermicularia, which were 

 attached to the testa, and which did not disappear with it, be- 

 came, as they are at the present day, adherent on the internal 

 mould. The proof that they did not become attached on the 

 mould already formed, is found in many specimens of the in- 

 ternal moulds of ammonites ; in which the folded edges of the 

 last partition are soldered, without any intermediate substance. 



