THE FRESH-WATER CLAM 229 



Unionidae so numerous as in the United States. They show 

 in our country a most marvellous variability also, so that hun- 

 dreds of Ivinds have been described from our waters. The 

 abundance of Unios in North America is due to the fact that 

 nowhere else is there such a large area of soluble Imiestone as 

 in our Mississippi Valley. The clams take from the water 

 the lime which they use in making their shells, change it into 

 an insoluble form, and thereby reduce the amount of the 

 inorganic matter in solution, and so render it more fit to 

 drink and to sustain other animal and plant life. 



Although numerous and of large size, Unios do not seem to 

 be much used as human food. The aborigines, however, 

 made use of them, as the great shell-heaps on the banks of 

 rivers of the Ohio Valley testify. They yield also pearls, 

 which occasionally, especially in the Ohio Valley, are of pre- 

 cious quality. Even the small, imperfect pearls are of value, 

 since they are ground up to make the powder used in polishing 

 the more valuable pearls. 



The spawning season of our Unios is short. The eggs pass, 

 during the latter part of October, into the gills of the parent, 

 where they undergo their early development, protected by 

 the parent shell and supplied richly with oxygen. The cleavage 

 of the egg is into unequal spheres because of the larger amount 

 of yolk in some of the cells than in the others. Eventually 

 a shell is formed, which divides into two valves united by a 

 straight hinge. A spine is formed on the free edge of each 

 valve, and a thread (" byssus ") is secreted from the body. 

 The young (Glochidium, Fig. 217), in the spring, become free 

 from the mother and attach themselves to the gills or fins of 

 a fish by means of the spines on the shells. Here they become 

 transformed. The single adductor muscle disappears, and 



