WORK DONE BY THE OYSTER 245 



are least favorable for their growth — these mollusks are rarely 

 very efficient agents in shallowing the water ; yet, even about 

 Boston, Massachusetts, certain of the estuaries, particularly the 

 part of the Charles River known as the Back Bay, has some 

 hundreds of acres of its area occupied to the depth of several 

 feet by the shells of this species. For many years the living 

 forms have disappeared from these waters, but it is evident 

 that a considerable part of the accumulations which have served 

 in good part to close the original entrance of Boston Harbor 

 are composed of their remains. 



South of New York the importance of the oyster in the 

 history of the harbors steadily increases until we attain the 

 coast of Florida. In its maximum development from near the 

 mouth of the Savannah River to Jupiter Inlet the larger part 

 of the shallow bottom inside of the ocean beach is occupied 

 by beds of these shells. So crowded are these forms that they 

 push their growth above the level of low-tide mark and in the 

 region where the mangroves abound they cluster on the roots 

 of the trees in such numbers as often to hide them from 

 view. The waves break off great quantities of shells and 

 toss them upon the beach which borders the widespread marine 

 marshes of this district, the so-called savannas of our southern 

 coast. In this position the debris is rapidly covered by the 

 swiftly extending vegetable growth. Between Charleston, 

 South Carolina, and Biscayne Bay, Florida, there is an aggre- 

 gate area of nearly a thousand square miles which, when the 

 shore assumed its present elevation, was occupied by tolerably 

 deep water that has now become filled to near the level of high 

 tide by sediments composed in large part of oyster shells. 

 The singularly rapid growth of these savannas as compared 

 with the marine marshes on the more northern parts ot our 



