CHAPTER 11 
MUSSELS AND PECTENS 
And now our investigations will take us away 
from the sea for a little while, and we will leave 
its rolling waves, its stretches of sand, its high 
cliffs, and its deep, still, cold abyss. We will make 
our first trip to the rivers and lakes, and we shall 
find them inhabited by mollusks also, though it 
will be easy to distinguish them from the creatures 
that live in the sea. Throughout our broad land 
the great streams abound in what are often known 
as river-mussels. They are especially numerous 
in the tributaries of the Mississippi, for those 
streams largely flow over limestone formations, 
which furnish material for the shells of the mol. 
lusks. They are not so common on the West Coast, 
for the probable reason that the water is not so 
hard as it is on the other side of the mountains. 
For many years the mussels were seldom dis- 
turbed, but of late years vast numbers of them 
have been collected for their pearly shells, out of 
which innumerable buttons are cut and turned. 
Oceasionally a valuable pearl is found inside one 
of these common looking shells. 
The first species to be mentioned in this list is 
Margaritana margartifera, Uinn., the Pearly 
River-mussel. In the upper Sacramento River, 
near Redding, they are numerous and grow to the 
