103 



fluctuations of August and the following months. The year 1898 

 was one of unusual irregularity in the hydrograph (Pt. I., PL XII.), 

 especially at the lower stages of the river, at which times this 

 rhizopod appeared most frequently. Its maximum occurrence, 

 20,898 per cubic meter on Jan. 25, accompanied a rise" of 0.6 of a 

 foot in 24 hours. At other times the numbers range from 100 to 

 8,000 per cubic meter, their irregularity affording additional ground 

 for regarding this species as an adventitious planktont. 



Cochlio podium was present in water ranging from 32.1 to 89, 

 the maximum number observed being found in water almost at the 

 freezing point, when the river was full of running ice. That this 

 is the optimum temperature for this organism is not, however, to 

 be inferred, since, as has been shown above, this species is adventi- 

 tious in the plankton. Plankton collections do not afford adequate 

 data for determining the seasonal cycle of the organisms habitually 

 living upon the bottom. This species was not found, though careful 

 search was made for it, in the winter collections of 1899. Its 

 absence from the records of years previous to 1898 may in part be 

 due to a failure to observe it in the silt-polluted collections in which 

 it is most apt to occur. 



Cyphoderia margaritacea Ehrbg. Average number, 198. This 

 species has occurred in every month but February. In 1898, the 

 majority of the occurrences and three fourths of the numbers ap- 

 peared between May 1 and October 1 at temperatures above 60. It 

 was never abundant at any time, though there is this indication of 

 its increased numbers during the warmer season. It is not an im- 

 portant element in our plankton. Apstein ('96) found it somewhat 

 irregularly in the plankton of German lakes. In our waters it 

 exhibits no marked dependency upon floods for its presence in the 

 plankton, though it is probably capable of assuming the limnetic 

 habit in the warmer season. 



Cyphoderia trochus Penard appeared occasionally with the pre- 

 ceding form, from which it is distinguished by its conical horn on 

 the fundus and by its larger scales. 



Difflugia. 



This genus is the most abundant one of the Rhizopoda in the 

 plankton of the Illinois River, and is a factor of quantitative 



