108 



XL) was only a fall of .1 and a rise of .2 of a foot changes due to 

 wind and the operation of the locks in the dams at either end of the 

 pool. The estimated percentage of silt is near the minimum from 

 a trace to 5 per cent. and the turbidity was no greater than is 

 customary (Pt. I., Table III.) in our waters during periods of abun- 

 dant plankton such as this (Pt. I., PI. XI.). Beyond the presence 

 of these rhizopods there was nothing in the plankton to suggest 

 that the bottom had been stirred up any more than usual. No 

 environmental factor is apparent to which we can attribute this 

 wave of Difflugia in the plankton. It is due, I believe, to their own 

 physiological condition. This was a time of prolonged low water 

 and great sewage contamination, and of remarkable development of 

 water-bloom, chlorophyll-bearing flagellates, unicellular algae, and 

 some diatoms, all elements in the food of Difflugia. In the open 

 water Difflugia could find abundant sustenance and thus maintain 

 itself there. It is not strange, then, that we find it in these warm 

 waters, richly charged with its food, assuming for the time a eulim- 

 netic habit, perhaps as a result of rapid growth and lighter shells, and 

 of increased metabolism with reserve products which lighten the 

 specific gravity and so facilitate flotation. 



This species is found throughout the whole range of temperatures,. 

 There are indications that its optimum lies above 60, and perhaps 

 near the maximum, 80. This may, however, be the result of the 

 effect of temperature upon the food supply of the organism. In any 

 case the plankton data can not suffice to follow the complete seasonal 

 cycle of an organism which is either an adventitious or but a tem- 

 porary constituent. 



