406 



Many variations in the nature and form of the test have been observed. 

 The studies of Penard ('02), Averintzev ('06) and others have resulted in 

 more than forty species being referred to this genus. The many variations 

 observed in the Difflugia in this limited habitat make evident the value of 

 studies on the effect of age and environment upon the form of the test. 

 Such studies would certainly define the species more clearly than they are 

 at present. The difficulties of such experiments are obvious. 

 Lesquerensia spiralis Schlumberger. 



Rare. 

 Pontigulasia compressa Carter. 



Nov. 9, 1909. 

 Arcella vulgaris Ehrenberg. 



This species was very common on the bottom and in the vegetable debris 

 during the year 1909 but it was very much reduced in numbers the next 

 year. In the collections taken with the pump from Jan. 5 to Aug. 12, it 

 occurred but once. 

 Centropyxis aculeata Stein. 



Occurred rarely. Taken Jan. 5, 1909. 

 Actinophrys sol. Ehrenberg. 



It was not found until May 28, 1910, when the water temperature was 

 20° C. It was quite common on that date and during the following month. 



Flagellata — 

 Euglena viridis Ehrenberg. 



Always present, but reaching its maximum development in Aug., '10, 

 when 27,560 per 100 liters of water were taken by filtering with No. 20 

 bolting silk. This filter undoubtedly allows some to pass through. 

 Phacus pleuronectes Miiller. 

 Phacus pyrum Ehrenberg. 



Both species were present among the filamentous algae at all seasons 

 but never in great quantity. The former was much the more common. On 

 account of their association with the alga? they wore always moi-e plentiful 

 in the margins of the pond. 

 Peridinium tabulatum Ehrenberg. 



A form that was referred to this species was observed in some ma- 

 terial brought into the laboratory Jan. 18, 1910. This material consisted 

 of debris and water. It was kept In a clean glass jar covered with glass 



