THE FRESH-WATER CLAM AND ITS ALLIES 179 



in the current of the water. A part of this current is 

 carried through the mantle chamber and forced out again, 

 deprived of usable food and of the oxygen used in respira- 

 tion. By devouring organic matter these clams act as 

 useful scavengers of the water. 



The family of Unionidae, to which Unio and Anodontia 

 belong, is of world-wide distribution, but nowhere else are 

 Unionidse so numerous as in the United States. They 

 show in our country a most marvellous variability also, so 

 that hundreds of kinds have been described from our waters. 

 The extraordinary abundance of Unios in North America 

 is due to the fact that nowhere else is there such a large 

 area of soluble limestone as in our Mississippi valley. The 

 clams take from the water the lime which they use in the 

 construction of their shells, change it into an insoluble 

 form, and thereby advantageously reduce the amount of 

 the inorganic matter in solution, for this change in the 

 quality of the water renders it more fit to drink and to 

 sustain other animal and plant life. 



Although numerous and of large size, the Unionidse 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 precious 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 Unionidse 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 is unequal because of the larger 



