352 



A quatic Societies 



still deeper water there are apt to be beds of Chara and 

 of pondweeds. These and the molluscs associated 

 with them, leave their calcified remains deposited on 

 the pond bottom as a stratum of marl. Thus the 



Fig. 2og. Mud pond, near McLean, N. Y. This is a bog pond, surrounded 

 in part at least by floating sphagnum. The outlet (to left in the picture) is 

 bordered by tussock sedges, backed up by extensive alder thickets. 



{Photo by John T. Needham.) 



filling of a bog pond is in time accomplished by the 

 deposition of a layer of marl over its bottom, and a 

 much thicker mass of peat over the marl. Successive 

 stages in the filling process are graphically shown in 

 Dachnowski's diagram, copied on the next page. 



Peat formation and filling of beds goes on, of course, in 

 ponds where there is no sphagnum; goes on wherever 



