ON MOLLUSCA. 267 
calcareous or corneous, flat or slightly concave, formed of 
concentric elements, and attached to the posterior part of the 
foot of the animal. This is named operculum, cover or lid. 
Its form and size are taken into consideration, but give rise 
to no peculiar appellations. It is not so with the manner in 
which itis joined to the aperture. Those opercles are simple 
which have no other relation than that of their form with the 
aperture of the shell; composite, which are articulated by 
means of eminences and of corresponding cavities. 
The bivalve shells, which we must very briefly dispatch, 
may be considered pretty nearly under the same relations as 
the univalve, and a few peculiar to themselves. As to the 
relation of the places where they are found, they are divided 
into fluviatile and marine. No terrestrial bivalves have as 
yet been discovered. 
The fluviatile are not very numerous, and perhaps are still 
more difficult to distinguish from the marine than the uni- 
valves. It may be remarked, however, that usually nacreous 
in the interior, they are covered with a thick epidermis of a 
green more or less deep, and that the summits are worn, or 
what is technically termed decorticated. None are yet 
known but among the species with a double muscular impres- 
sion, and altogether closed or short. 
The next relation under which the bivalves are considered 
is that of their fixity, or that of their mobility ; but of this we 
have already spoken sufficiently in another place. 
A third relation under which the bivalves may be consi- 
dered is whether they are free or concealed in a tube more or 
less developed. In this last case the valves are altogether 
contained and concealed in a tube of the same nature as them- 
selves, and open only at one of its extremities. ‘They may be 
called tubicole. ; 
Another point of view is that of the substance in which 
they may be found: but this has been already treated of. 
