106 
ROOM XIII. 
Nat. Hist. 
ing the tip bending in the same direction. Many of 
these animals, during their torpidity, cover the mouths 
of their shells with a membranaceous or calcareous case, 
which is dissolved or thrown off when they revive. 
The family of the Cyclostomida differ from all the other 
Land Mollusca, in having the respiratory cavity open in 
front, and in not being hermaphrodite: they have subulate 
contractile tentacula, with the eyes at the base, like the 
pond-snails. They are the only land shells which have 
an operculum ; this family contains the genera Cyclos¬ 
toma and Helicina. 
The Cases 66 to 86 contain the second class of Mol- 
lusca, which have bivalve shells, and whose animals are 
always covered with a two lobed mantle, each protected 
by a shelly valve, and they have within the mantle, be¬ 
tween it and the compressed body, a pair of laminar 
branchiae on each side. The lower part of the body is 
generally dilated into a keeled or horn-shaped foot, by 
which they walk along the sand or mud of the shore, or 
a flat disk, by which they attach themselves to rocks 
and form holes in their surface. They are all aquatic 
and are divided into orders, according to the structure 
of the mantle. In some of these, the elastic cartilage 
by which the valves are separated from one another, 
when the muscles which close them are relaxed, forms 
an external band along the hinder edge of the shell, 
between the shell and the ligament by which the two 
valves are fastened together; as in the genera Venus, 
Tellina , Cardium , and Solen. In the last genus, the 
cartilage end ligament are very prominent, and_ the 
ridge on the margin of the shell from which it arises, 
and against which the cartilage is pressed by the liga¬ 
ment when the valves are closed, is very large and dis¬ 
tinct. In others, as the Mactrce and Crassatellce , the car¬ 
tilage is placed in a small triangular cavity, situated just 
at the back of the teeth, and the longitudinal fibres, 
of which it is formed, are pressed by the surface of 
the valves when they are closed; these shells have 
the ligament placed exactly as in other bivalves. In 
some 
