244 ^^A A^'D LAND 



ments. There is a very great diversity in the mode of life 

 adopted by these molluscan species. The most of the uni- 

 valves are unattached to the bottom, and are apt to be swept 

 about by the tidal currents where the movements are partic- 

 ularly strong. The bivalve shells which are prevailingly 

 laro-er and make more considerable accumulations of detritus 

 are usually fixed to the bottom or dwell in the mud, in which 

 they excavate chambers large enough to give shelter to their 

 bodies. Some species which lie upon the floor of shallow 

 water are not attached, and at their death their shelly cover- 

 ings are often w^ashed ashore. In some cases the accumulation 

 made along the margin of the harbor is sufficient in quantity 

 to lead to the formation of a shelf, which gradually extends 

 toward the deeper water. Generally, if not always, this accu- 

 mulation of shells is more or less associated with vegetable 

 matter formed by the marine marsh plants. 



By far the most important of our mollusks in the work of 

 accumulating sediments in harbors is the common oyster, which 

 exists in the form of numerous ill-determined species along the 

 greater part of the shores in the temperate and warm portions 

 of the world. The most of the varieties of this interesting 

 animal have become especially adapted to life in brackish 

 water, such as our harbors afford. In general they require a 

 mixture of from one-fourth to three-fourths of water from the 

 land with that from the sea to afford conditions for their fullest 

 development. Where these conditions are combined with 

 shallow water the oysters may grow rapidly, their shells form- 

 ino- an almost continuous sheet over wide areas, the living lying 

 upon the dead in a deposit many feet in thickness. In the 

 northern part of the field which they occupy — as, for instance, 

 between New York and Newfoundland, where the situations 



