First, we see that the shell is composed of two parts, called 
valves, fastened together at the back by a strong but flexible 
ligament. Below this, the hinge has several prominences, 
called “teeth” which fit into corresponding cavities in the 
opposite valve. These strengthen the hinge and make it open 
and close accurately. Above the hinge, projects the “beak ” 
or umbo of the valve. At each end of the shell there is a 
scar (A, P) marking the places where two muscles are 
attached. These muscles are called “adductors” because 
they close the shell. The line connecting the sears of the 
adductors is where the mantle or lining of the shell is attached, 
and is called the 
pallial line. The 
edge of the man- 
tle extends to the 
edge of the shell, 
and gradually 
Fig. 2. Hinge of Beach clam, Mactra solidissima. buildsthelatter 
up by depositing thin layers of lime. A bay (or pallial sinus) 
in this line indicates that the two siphons or tubes through 
which the animal draws water to the gills, and expels it, are 
long, as in the common sand clam. 
In some bivalves there is no ligament on the outside, it being 
transformed into a cushion of cartilage which occupies a pit 
or cavity below the beaks. Such a one is Mactra (Fig. 2.) 
Univalves, or spiral shells, called GASTEROPODs in scientific 
language, have the shell in one piece, usually in the form of a 
spiral cone. The opening or aperture is generally round or 
oval, and has either a continuous even margin, or a notch 
