ON THE STRUCTURE OF CAMERATED SHELLS. 
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No siphuncle, however, exists in any of these shells. 
The shells of all Mollusca are composed of carbonate of 
lime ; but, so different is the mode of its arrangement in the 
several orders, that their texture is as various and characteristic 
as that of arragonite and calcite, although, like these minerals,, 
they have precisely the same chemical composition. 
Some, when broken, present a dull lustre, like marble or 
china, and are termed porcellanous ; others are pearly, or 
nacreous ; some have a fibrous structure ; some are horny ; 
others are glassy and translucent . 
In Nautilus the inner layer and septa are nacreous ; the 
outer layer is porcellanous. The composition of the shell in 
Nautilus is, then, the same as in the other Mollusca. As in 
them, also, it is coated, when alive, with a layer of epidermis 
or periostracum , which is not a living membrane, and can only 
be reproduced around the mouth of the shell, or where it is 
within reach of the margin of the mantle. It is the umbonal 
portion of bivalve shells and the spires of univalves which first 
become eroded and injured; and one object, no doubt, in the 
formation of septa in all shells, is to shut off the damaged and 
untenantable part of their abodes. 
Dr. S. P. Woodward, in his “ Manual of the Mollusca ” 
(p. 82), observes : “ With respect to the purpose of the air- 
chambers , much ingenuity has been exercised in devising an 
explanation of their assumed hydrostatic function, whereby the 
Nautilus can rise at will to the surface, or sink, on the approach 
of storms, to the quiet recesses of the deep. Unfortunately for 
such poetical speculations, the Nautilus appears on the sur- 
face only 6 when driven up by storms ,’ and its sphere of action 
is on the bed of the sea, where it creeps like a snail, or perhaps 
lies in wait for unwary crabs and shell-fish, like some gigantic 
sea-anemone,, with outspread tentacles.” 
Mr. Frederick Edwards, in his Monograph of the Eocene 
Cepbalopodous Mollusca (Palaeontographical Society, 1849, 
p. 12), says: i; It is obvious therefore that the hydrostatic 
balance would be destroyed if any one of the deserted chambers 
were so injured as no longer to act as a float.” 
In Woodward’s ct Manual of the Mollusca” we find it also 
stated that “ the use of the air-chambers is to render the 
whole animal, and shell, of nearly the same specific gravity 
with the water” (p. 82). 
But no such buoy would be required for a bottom-feeder ; 
indeed, it would prevent it from remaining below. I believe 
the facts of the case tend to show that, like the “ water- 
Spondylus,” the chambers were filled, or partially • filled, with' 
sea-water, which would be certain to find its way into the 
chambered portion of the shell, through its pores, thus 
