166 SUPPLEMENT 
The shells of oysters contain much less animal matter, and 
this matter resembles more a gelatinous substance. M. Vau- 
quelin has found there, besides the organic matter, some sub- 
carbonate and phosphate of lime, subcarbonate of magnesia 
and oxide of iron. 
The shell of the patelle, which presents a very close la- 
mellated structure, approximates still more in its chemical 
composition to those whose structure in general is vitreous. 
The latter, according to Mr. Hatchett, who names them por- 
celaine shells, contain but a very smali quantity of azotic 
matter; we find there, on the contrary, much subcarbonate of 
lime, but without any traces of phosphate or sulphate of the 
same basis. 
After what we have now said, it is evident that the shell of 
the wolluscous animals, a mucoso-cretaceous substance, is not 
a hardening of the skin by the deposition of calcareous mole- 
cules in the meshes of a cellular tissue, but truly a deposition 
of mucoso-calcareous matter, not however excreted at the su- 
perficies of the skin, but between two of its parts, the vascular 
net-work and the epidermis, and sometimes even in the dermis 
itself; and, in fact, it is connected organically with the rest 
of the animal, and especially with the muscular or contractile 
fibre, while a simple calcareous tube, like that which exists in 
the tubicole, is in reality but a deposition, an exhalation alto- 
gether external, and accordingly is not, properly speaking, 
attached to any part of the animal. It is this point of relation 
of the animal with the shell which produces the impressions 
of variable form which are remarked in different parts of the 
shell, and especially in the bivalves. ‘This necessary relation 
then does not permit us to suppose that a conchyliferous mol- 
luscous animal, whose shell had been removed, could re- 
produce it, still less that it could quit it of its own accord, as 
Bruguiéres supposed in the case of the porcelaine shells. 
Neither does it permit us to adopt.the notion of Klein and 
a 
