The Cyrenas and Pea Shells 



tongue-like foot slips in and out of the broad end of the shell. 

 Length, J inch. 



Habitat. — River margins, ponds and lakes, Utah, Cal- 

 ifornia, Oregon. 



Genus PISIDIUM, Pfr. 



Shell somewhat pea-like in size and form, thin, smooth, with 

 united siphons. Similar to Sphaerium, but smaller; valves not 

 so nearly equilateral. Habits similar. 



The Hidden Pea Shell (f. ahditum, Hald.) inhabits brooks 

 and other fresh water, where it is difficult to discover, so well does 

 its thin brown shell blend with its surroundings. Besides, it is 

 scarcely larger than a grain of rice and may be easily mistaken 

 for a seed of some water plant. It ranges widely, from New 

 Mexico to the northwestern states. Modified forms have received 

 varietal names, and many have been called new species. Several 

 other pea shells occur in this country. There are several species 

 in Europe. All are interesting tenants of the aquarium. 



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