160 SUPPLEMENT 
may be heated very considerably without breaking, which is 
the reason why the first of them is often used as a sort of 
dish. 
Sometimes, when the calcareous molecules are depositing 
themselves in the formation of one of the composing lamine, 
they correspond or place themselves one above the other in all 
those which compose the shell, and from this results the 
fibrous structure, in which.the sbell is more easily broken in 
the direction of the fibres than in that of the lamine. 'Thisis 
often very observable in the shell of the pinna. 
We find some shells in which these two structures may 
alternate; that is, one part of their thickness is simply 
foliated, and the other fibrous. This is the fibro-lamellary 
structure. 
A structure much approaching to this is that which we 
remark in nacre-shells, or those which produce mother-of- 
pearl, whether univalve or bivalve. The nacreous part appears 
to be always lamellated, and the other to be fibrous, or more 
or less oblique. 
When a shell is arrived to the degree of size of which it is 
capable, the dermis of the animal appears to produce a greater 
quantity of calcareous matter and less mucous matter, and 
the molecules which compose it are no longer deposited by 
lamine, or regular strata. They are very much crowded, 
heaped together, and assume a vitreous structure, which is 
polished more and more with age, by the rubbing of the parts 
of the mantle, which may be remarked in all the univalve 
shells, at their internal surface, and especially near the 
aperture, as in galea, for example; but it is seen still better 
in the porcelaines, and some neighbouring genera, where, 
in consequence, the animal being provided with two large 
lateral lobes to its mantle, the shell is almost every where 
enveloped by it. 
