ROOM XIII.] NATURAL HISTORY. 91 
and the compressed body, a pair of laminar branchiae 
on each side. The lower part of the body is generally di ¬ 
lated 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 se¬ 
parated 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 and 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 ligament 
when the valves are closed, is very large and distinct. In 
others, as the Mactrce and Crassatellce , the cartilage 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 few, as the Pid- 
docks ( Pholas ), there is no cartilage, its place being sup¬ 
plied by muscles, which are attached to the posterior edge 
of the valves, which are covered by a thin skin instead of 
a ligament, in which shelly plates are usually imbedded. 
The animals of bivalve shells are in general free, and walk 
about by means of their compressed foot, forming for them¬ 
selves holes in the sand or mud on the sea-coast, in which 
they rest with their syphons near the surface, and their 
mouths downwards. Others, as the Petricolce, Lithodomi , 
and Pholades, form for themselves holes in calcareous 
rocks or old shells, in which they constantly remain dur¬ 
ing the whole of their lives. Some few line these holes 
with a calcareous secretion, as the Gastrochcence and Tere¬ 
dines. The Clavagella and Aspergillum form testaceous 
tubes, to which the former fixes one of its valves, leaving 
the other free to move at the will of the animal, while the 
latter fixes them both, so that the valves appear to form 
a part of the tube, their apices only being visible exter¬ 
nally. Those animals which fix the valves to their tubes. 
