318 SUPPLEMENT 
pears that the animal can shelter the anterior part of its body 
under the advancement of the upper lamina over the lower, 
but it can never draw back into the shell itself. Perhaps even 
in a state of repose, the animal envelopes its shell with the lobes 
of its mantle, as is the case with some other mollusca. 
When this shell has been removed, (which cannot be done 
but by breaking it, if we wish to preserve the animal, because 
the entrance of the shell is much more narrow than its cavity,) 
we see more evidently the separation of the body into two 
parts. Both one and the other are perfectly symmetrical. 
The posterior or abdomen presents the very form of the 
sheath or shell, which it fills exactly. Accordingly, itis much 
more plane above, and, on the contrary, considerably more 
curved underneath. It is entirely enveloped by a mantle 
very thin in the middle parts, where it is adherent, and, on 
the contrary, more thick in the whole circumference which 
borders the cleft of the shell, and which is more or less free. 
Above it is prolonged like the upper plate of the shell; un- 
derneath it equally edges the inferior lamina, but there it is 
thicker. It becomes more especially so on the sides, where 
it edges the cleft of the shell, and where it is divided into two 
lips, but which, nevertheless, are not cleft in their length. 
At the posterior extremity of their union, there seems to exist 
a sort of strip, which is only their prolongation, and which 
may sometimes be much longer than the shell. 
In the space which separates the two united lips from this 
lateral part of the mantle, is formed a sort of fold, or projecting 
plate, equally muscular. 
From what Forskal says of the edges of the mantle, it ap- 
pears that in a living state they have the capacity of extend- 
ing very considerably, and become very thin, and even trans- 
lucent. 
From this description of the mantle, it follows, that it is 
really open only in all its anterior part, especially above and 
ee 
