X.— OYSTER SHELLS (Two-fifths Natural Size). 

 Contrast these with the shells shown in Plate XI. These shells are from 

 oysters grown in brackish water, where there was but little sediment, and 

 then transferred to salt water, where there was practically no sediment. 

 Note that on two of them there are traces of sediment streaks which the new 

 layers of shell deposited in the clear water have not covered entirely. 



