84 WESTERN SERIES OF READERS. 



But besides all these creeping water-snails, and 

 many others like them that I have not mentioned, 

 there is another kind of mollusks with bivalve 

 shells, which are sometimes found living half- 

 buried in the sandy bottoms of rivers and lakes. 

 They are often called " Fresh-water Mussels/' 

 though they are very different from the mussels 

 of the sea. 



Two species, of about the natural size, are 

 shown in Figures 38 and 39. Those of you who 

 live near a lake or a stream may be able to find 

 some of them, or perhaps other kinds. I have 

 been told that they live abundantly in the Sacra- 

 mento River, after it leaves the mountains and 

 becomes quiet. 



Most of their shells on this coast are rather thin 

 and brittle, on account of the lack of limestone in 

 our mountains; but in the Mississippi Valley they 

 are very numerous and strong, and inside these 

 "Unio" shells are sometimes found beautiful 

 pearls. Great numbers of these shells are now 

 gathered and made into buttons and other arti- 

 cles of pearl. 



There are also many small bivalves living in 

 some streams. Some of their full-grown shells 

 are not so large as a pea, but the little mol- 

 lusks living within them are active fellows, and 



