18 WEST AMERICAN SHELLS 
from falling apart, and beside it there are more or 
less hinge-teeth for the same purpose. The lateral 
tooth is marked ‘‘]. t.’’? in the picture, while the 
three cardinal teeth are situated just below the 
umbo, which is marked ‘‘u.’? One more mark, 
namely, ‘‘lu,’’ signifies lumule. This is a heart- 
shaped depression on the outside of the shell, half 
in each valve, and is seen when you look at the end 
of the shell, as in Figure 56. 
It is interesting to capture a healthy clam and 
put him in a jar of sea-water with a thick bed of 
sand at the bottom, and see him adjust himself to 
the changed conditions. He digs with his foot, 
and he pumps water through his siphons. His 
pumps, however, are invisible, for in reality they 
consist of innumerable little lashing hairs, or 
cilia, covering the surface of the gills. A bit of 
gill may be snipped off from a freshly opened 
oyster or clam and placed in a drop of sea water, 
under a microscope, and the movement can be 
plainly observed. 
Although the clam has no head, the part which 
goes down into the burrow first is called the front 
end, and the siphons always follow. In the dead 
shell the pallial sinus is therefore always at the pos- 
terior end. When a shell is in position for deserib- 
ing, as in the figure, the ligament is at the top. 
It is easy to see that Figure 59 represents a left- 
hand valve, for the position of the sinus is plain. 
In describing the shells of peleeypods we shall 
have oceasion to eall attention to all of these fea- 
tures, also to the epidermis, or periostracum, 
