300 WEST COAST SHELLS 



north of Yosemite Valley, and the Coast Ranges 

 south to San Francisco Bay. The Coast Range 

 System includes the coastal drainage south to Point 

 Conception, the Sacramento-San Joaquin \'alley, 

 and the west slope of the southern Sierra Nevadas. 

 The Los Angeles System includes the coastal drain- 

 age of southern California south to the San Pedro 

 Martir Mountains in Baja California. The Colo- 

 rado System includes the drainage of the Colorado 

 River above The Needles. The Arizona System 

 includes the Gila and Salton Basins and the Colo- 

 rado Delta. 



THE NAIADS OR RIVER MUSSELS 



(Unonid^) 



In almost any large perennial stream or lake are 

 to be found members of this interesting group. They 

 are especially abundant both in species and indi- 

 viduals in the tributaries of the Mississippi River, for 

 those streams flow over limestone which dissolves and 

 forms abundant material for their shells. They 

 often attain a large size; some species are as big as 

 this book. Of late years an enormous industry has 

 developed of collecting these shells for the })urpose 

 of making pearl buttons and as a result their num- 

 bers have been terribly depleted and laws are being 

 enacted to protect them. Since only large, heavy 

 shells are used, collectors on this coast need not 

 worry, for our species are too few in numbers, too 

 small, and too fragile. Pearls are often found in 

 these mussels; the writer has obtained them in both 



