440 FOSSIL INVERTEBRATED ANIMALS. 



stucco, or plastering, which appears to be the cause of their 

 thus detaching themselves from the mass. 



If we do not admit that the shells and other bodies become 

 changed into marl, it is very difficult to explain the formation 

 of these models — both the internal and external mould being 

 the same substance. Had there been a disappearance of the 

 testa of the shells, as in the other localities, it would be neces- 

 sary to suppose that a petrifaction had seized the bodies ; that 

 afterwards the marl was moulded ; and, finally, the mould 

 itself was changed into marl. But it must be confessed that 

 these transmutations are not very easy to be comprehended ; 

 and, perhaps, without straining at any better explanation, we 

 may be contented to conjecture that all the calcareous sub- 

 stances contained in the stratum have been converted into 

 marl. 



When the hipponyces are found in a stratum, where there 

 has been a disappearance such as we have been speaking of, 

 they present a very singular phenomenon. Their upper shell, 

 which is composed of a substance analogous to that of the 

 cyprsese volutae, and other soluble shells, has disappeared, 

 leaving only its mould ; while the under, which is of a foliated 

 texture, like that of oysters, remains untouched, with the ex- 

 ception of that part where the adductor muscle is found. This 

 organ, which is displaced, or at all events extended, in propor- 

 tion to the growth of the animal, has furnished, on the side of 

 the under shell or support, the same soluble matter which it 

 furnished to the extremity by which it was attached to the 

 upper, or shell proper 5 so that, when this last, and its thick 

 support, were in circumstances proper for their dissolution, the 

 shell, and the place of the support where the muscle was 

 attached, have alone disappeared, and the rest of the support 

 has remained untouched. 



The lirostrites (Lamarck), as well as the spherulites, exhibit, 

 in like manner, very singular facts in their petrifaction. Their 

 testa, or at least that of the lower valve of the first, the 



