176 RIVER PEARL-SHELLS. 



brown; shell pearly within, somewhat flesh -colored. 

 The length is three inches, which is twice the height. 



Margaritana margaritifera, Iyinn., Mar-gar- i-ta'-na 

 mar-gar-i-tif-e-ra. This fresh-water mnssel is widely 

 distributed, being found in the eastern part of the 

 United States, and also in Europe. The shape of its 

 shell is oblong, somewhat bulged at the umbones. 

 Firm and solid ; hinge- teeth strong, triangular. Epi- 

 dermis dark, internal color somewhat purple, length 

 three inches. Found in the Chehalis river of Wash- 

 ington, and the Shasta river of Oregon. 



Very much smaller than the fresh- water mussels 

 are the following little creatures which live with them 

 in the brooks and rivers. The first one is named 

 Pisidium compression, Prime, Pi-sid'-i-um com-pres'- 

 sum. The bivalve shells are minute, somewhat tri- 

 angular in shape, plump and full, covered with a 

 brown epidermis. In form and size they resemble 

 radish seeds. The length is one-eighth of an inch. 

 Specimens from the Columbia river, near The Dalles. 



Pisidium ultramontanum, Prime, ul-tra-mon-ta'- 

 num. Still smaller than the last species. Shell 

 thin, smooth, light brown. From Utah. 



Pisidium abditum, Hald., aV-di-tum, Fig. 149. 



Shell oval, thin, marked with minute lines of growth. 



Color light brown, length sometimes nearly 



><-% one-fourth of an inch. This little species 



is quite widely distributed, and its name 



Fig- 149- has numerous synonyms. Its variety occi- 

 dental, Newcomb, ok-si-den-ta'-le, is found in a lit- 

 tle stream running out of Mountain L,ake, near San 

 Francisco. This shell is usually about an eighth of 

 an inch long. 



