102 WEST AMERICAN SHELLS 
species. Within each valve, beneath the hinge, is 
a slender spoon of shell, very narrow and delicate ; 
its use is not fully known. On the outside, just 
above the ligament, is a long projecting plate with 
straight sides. This auxiliary valve, as it is called, 
is curved in front and straight behind. The shells 
gape widely at the ends. The length of the white 
valves is about two and a half inches. The crea- 
ture burrows in mud and clay. 
Penitélla penta, Conr., the Common 
Piddock, (Pholadidea’ penita), is 
shown in Figure 90. While it is often 
much smaller than the picture, speci- 
mens are sometimes found that are 
more than twice as long. Like the 
other Piddocks, the forward part of the 
shell is rounded and rasplike, while 
the latter part is narrow and smooth. 
A triangular plate covers the hinge- 
area, and the valves end in epidermal flaps or 
seales. It is commonly found in burrows which it 
constructs for itself in the softer rocks which occur 
here and there all along our coast. In young speci- 
mens the forward end gapes widely to allow the 
strong foot to press against the side of the burrow 
and thus assist in turning the shell, whose sharp 
points wear into the rock, though just the method 
by which it bores is not clearly known. As it 
grows older it seems to realize that it has gone 
far enough, and closes the front of the opening in 
the valves with a wall of shell, making them 
appear very round. 
