ON MOLLUSCA. 159 
biphorz, where they are so much separated that they seem to 
be at the two extremities of the body, one of them, the dorsal, 
is provided with a valvular apparatus. 
But a more singular character of the skin, in a great number 
of these animals, is, that in a part of its thickness, and most 
frequently between the vascular net-work and the pigmentum, 
a mucous matter is deposited, mixed with a greater or less 
quantity of cretaceous substance, the accumulation of which, 
when dried up, produces a protecting body, or in other words, 
a shell. 
We shall say something hereafter concerning the forms of 
shells, and those of their different parts, so as to draw from 
them the characters of this accessory branch of zoology. Our 
business at present with these bodies is as to the relations of 
their structure, chemical composition, the manner in which 
they originate, grow, and are modified with age; and finally, 
of their connexion with the animal. 
A true shell is always composed of mucoso-calcareous 
strata or laminz, applied one inside the other, the oldest and 
smallest being outside, and the newest and largest being the 
last inside. ‘This is evidently observable in the foliated 
shells, such as oysters, especially, when by exposure to the 
heat, or by long action of the air, the mucous matter which 
connected not only the molecules of each lamina, but also those 
of the two superposed ones, has been removed. The edges of 
the composing laminz, which are seen at the external face of 
the shell, constitute what are named the strie of augmenta- 
tion. 
This structure, the best known of all, is the foliated struc- 
ture. But there is another which differs from it, by the com- 
posing strata being much better connected, and their calca- 
reous molecules more approximated together: such is that of 
the shells of pecten and patella. Accordingly, these shells 
