74 PEARLS AND PEARLING 



button material. Other shells that are good material for 

 making buttons in one river may be badly stained in an- 

 other place, so their value would be reduced. 



There are a few shells that can usually be depended 

 upon in nearly all rivers in which they are found, but 

 they are not all found in all rivers. In considering the 

 various kinds of shells that are of value commercially it 

 should be remembered that while only a few are of value 

 for economic uses, practically all varieties are pearl bear- 

 ing. 



In some rivers, such as the Shell River in Minnesota, 

 even the thin shells commonly known as "paper shells," 

 or "floaters," are almost thick enough to cut button 

 blanks from, so even they might possibly yield some 

 pearls. As a rule pearl hunters do not separate the .un- 

 saleable shells from the valuable ones until after they 

 have "cooked out" the mussels. In this way they secure 

 all or most of the pearls, baroques and slugs that happen 

 to be in the shells. 



Each variety of mussels has a special scientific name 

 by which it can be identified in all instances. Each 

 variety of the mussels has also one or more common 

 names in various localities, but the different localities 

 often have different common names for the same mus- 

 sel, so the common names can not always be depended 

 upon to identify any particular variety of mussel. 



For this reason it is necessary to use both scientific and 

 common names in mentioning the different varieties. 



Fig. 8 shows The Quadrula heros, or "washboard," 

 and is about one-third natural size. This is found in 



