232 Our Food Mollusks 



forming tidal flats that may be partially or completely 

 covered with oysters (Figure 49). 



Islands are formed in many of the channels, often so 

 numerous that there remain only narrow streams be- 

 tween them. Though these rise only a few inches above 

 the high water line, they are frequently covered with 

 grass in the center. Around their margins are oyster 

 fringes, here, as elsewhere, extending from the low to 

 the high tide level. 



It has recently been shown by some very interesting 

 observations conducted by Dr. Caswell Grave, that these 

 islands are formed by the oysters themselves. In the 

 beginning of the process there may have been an open 

 channel, the river banks alone being fringed with 

 oysters. A river bank, of course, does not lie in a per- 

 fectly straight line. Here and there parts of it project 

 into the current. Now oysters growing on such points 

 are very much favored, because the currents are a little 

 more rapid there. This means that aeration is better, 

 and especially that a greater amount of food is brought. 

 Consequently these favored oysters grow more rapidly 

 than others. Their number, also, increases more rap- 

 idly here, for the reason that the more active current 

 keeps their shells comparatively clean, thus affording ob- 

 jects for the attachment of the young. A greater number 

 of swimming embryos, also, will be brought to the pro- 

 jecting point by the swifter current than to other parts 

 of the river bank. From time to time, winds, waves or 

 ice, break clusters of oysters from the point. They roll 

 down toward the channel, many of the oysters continu- 

 ing to live and grow. Year after year these clusters ac- 

 cumulate on the bottom beyond the point. Sediment 

 gradually settles among the lower shells, while new gen- 



