Forms, Composition, and Structure of Shells. 339 
tive proportions of these constituent parts. * Mr. Hatchett, 
to whom we are indebted for all our correct information on 
this subject, has divided shells into two classes, according to 
the proportion and state of their animal matter. The first 
class he names Porcellaneous Shells, since they resemble por- 
celain, are usually of a compact texture, and have an enamel- 
led surface, which is often finely variegated. The convolute 
shells afford good examples of this class. They consist of 
carbonate of lime, cemented together by so small a portion of 
albumen, that, when el in a dilute acid, the shell is 
completely diccoly ed, and not a sensible trace of it left behind. 
* The shells belonging to the second class are usually covered 
with a strong epidermis, below which lies the shell in layers, 
and composed entirely of the substance well known by the 
name of mother-of- pearl. They have been distinguished by 
the name of Mother-of-pearl Shells.” The fresh-water mus- 
cles, the oysters, the Naliotis, and pearly ‘Tarbines are 
examples of ‘this class. ‘* When immersed in acids they effer- 
vesce, at first strongly, but gradually more and more feebly, 
till at last the emission of air-bubbles is scarcely perceptible. 
The acids take up only lime, and leave a number of thin 
membranous substances, which still retain the form of the 
shell.” ‘These membranes have the properties of coagulated 
albumen.+ But the distinction between these two classes 
holds good only in extreme cases; for there are many shells 
which are intermediate, and stand on debatable ground, The 
compact bivalves dissolve in the menstruum entirely, as does 
also the common whelk (Biccinum undatum), but they are 
not properly porcellaneous; while the various land snails 
leave an insoluble membrane, though they are not perlaceous 
shells. 
Whence doth the animal borrow these materials, and in 
what manner doth it apply them to rear up its mansion well 
fashioned in every part? To the first question, I believe, no 
satisfactory answer can be given. Some physiologists indeed 
have analy sed the food, and have summed up its constituent 
item of lime, and there find the earthy material of the shell. 
But the quantity of lime which the animal takes in this w ay 
seems trifling when compared with what is required, and 
which it actually excretes. I know that, in favour of the opi- 
nion I combat, it has been asserted that the edible snail (Hélix 
pomatia), which under ordinary circumstances forms a calca- 
* Raspail and Prevost have found that bivalve shells in the first periods 
of their growth consist wholly, or nearly so, of phosphate of lime, a dis- 
covery as inexplicable as it was unexpected. 
|; Thomson’s Chemistry, v. p. 554-5. Edinburgh, 1807, 
