The Soft Clam 287 



roots, it is difficult to dig these tracts, but in such beds 

 there is preserved a great number of breeding individuals 

 that may restock neighboring beds rendered barren by 

 excessive digging. 



Mya, like the oyster and all other lamellibranchs or bi- 

 valves, feeds almost entirely on diatoms that are carried 

 in countless numbers by the brackish tide currents. 

 Other conditions being favorable, the number of clams 

 that may exist on any area, depends on the amount of 

 food that they are able to obtain. It is evident, also, that 

 the quantity of nourishment will depend on the amount 

 of water that passes over the bed, as Mobius, the Dutch 

 biologist, pointed out many years ago. Hence a tide 

 current passing over a bottom that is firm enough to be 

 undisturbed by it, is more conducive to rapid growth than 

 quiet water. This theoretical assumption has been 

 abundantly proved by experiments with Mya, as with the 

 oyster. Rapidity of current, within certain limits, de- 

 termines, in large measure, the number of clams that 

 may exist on a given area, and also the rate of their 

 growth. So, in selecting a bed for artificial culture, a 

 bottom should be chosen over which there is a free move- 

 ment of water. 



An interesting and suggestive condition existing on 

 most clam flats, is found in the close crowding of in- 

 dividuals over certain areas. If such a tract is kept un- 

 der observation, it will sometimes be found that, for 

 years, the numbers remain nearly constant, while growth 

 seems to have ceased, and there seem never to be any but 

 small clams. Food is sufficient for the existence of a 

 certain number, each apparently being able to get its 

 share, but none obtaining enough for growth. Such a 

 balance has often been observed. 



