XI. — OYSTKK .SHKJ^l^S. 

 These represent the type of shell produced by an oyster raised on a soft, 

 muddy bottom, where the sediment is continually being stirred up. Note the 

 streaks of sediment along the edge of the shell, that have been discarded by 

 the oyster. This condition cannot help but retard the growth of the oyster. 

 These streaks of sediment would be washeid away were the current strong 

 enough. Biit where they are not carried off, they, being irritating to the man- 

 tle of the ov.ster, are continually being covered over with new layers of shell, 

 till the condition is reached, as shown in the upper right-hand figure, where 

 the streaks are almost obliterated, and the water, having remained clear for 

 some time, no new streaks have been produced. 



