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., 710, 
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 alge at all seasons 
but never in great quantity. The former was much the more common. On 
account of their association with the alge they were always more plentiful 
in the margins of the pond. 
Peridinium tabulatum Elirenberg. 
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. 
