36 WEST AMERICAN SHELLS 
ing forward from the umbo. The ligament is long 
and strong, and the interior of the shell is tinted 
with purple. 
It is extremely interesting to note the different 
habits which members of the same family now pos- 
sess, and which they may have acquired long ago 
and passed down to posterity. The long, singu- 
lar shell shown in Figure 17 is evidently a kind of 
mussel, but its owner prefers a safe, sheltered re- 
treat to a mere anchoring place on the side of a 
wave swept cliff. The name of this species is 
Adula falcata, Gld., the Pea-pod Shell. Among 
the difficult things to explain is the fact that a mol- 
Fig. 17 
lusk, with a thin and flexible shell, can bore a deep 
hole into hard rock. But this creature does it, for 
I found the rocks of Duxbury Reef at Bolinas al- 
most alive with this and other borers. It spins a 
byssus, too, like other mussels, and attaches itself 
to the sides of its burrow. The figure represents 
a rather large specimen. The inside of the shell 
is white and pearly, while the outside is covered 
with a dark chestnut epidermis, which has num- 
erous transverse wrinkles. 
Adula stylina, Cpr., the Short Adula, is smaller, 
shorter, and has very angular shells with a brown 
epidermis destitute of distinct wrinkles. 
Lithophagus plimula, Hanl., the Rock-eater, has 
a small, cylindrical shell, rounded in front and 
tapering behind. It constructs a burrow some- 
