SHELLS OF LAKES AND STREAMS 301 



Gonidca and Margaritana. They are seldom of 

 value however. 



The Naiads have a very interesting life history. 

 The young, called glochidia, after being hatched in 

 the parents' gills are thrown out into the water in 

 vast numbers. The glochidium is a minute, curious 

 creature, armed with hooks, and hangs suspended by 

 a byssus from some aquatic plant. An unwarv fish 

 bumps against it; immediately the glochidium clings 

 to it by its hooks. It remains attached about three 

 months when it has grown a shell and become so 

 heavy that it falls to the bottom. Then it grows 

 into an Anodonta or whatever its parents were. 

 Many die to one which survives, but the Naiads are 

 prolific and keep up the species while this peculiar 

 method serves to distribute them. 



One warm day while on a collecting trip, I stopped 

 at the bridge over the San Lorenzo River below 

 Felton, Cal., and prepared to investigate the clammy 

 m5^steries of the river. This river is a splendid 

 stream, broad and clear, running between high banks 

 lined with willows, alders, and other water-loving 

 trees. After the dusty road it felt good to wade 

 along in the cool water and feel the clean sand-grains 

 sift across one's toes. Whoop I A startled bluej ay 

 flew screaming from a neighboring tree where he 

 had been suspiciously eyeing me. I bared my arm, 

 reached into the water, and picked up a dark object 

 partly buried in the bottom. Shaking the sand off 

 I held up a fine specimen of Margaritana margariti- 

 fera, Linne, the Pearly Naiad, Plate I, Fig. 4. It 



