43 



of the plankton, and perhaps with the result of increased photo- 

 synthesis in periods of lunar illumination, which tends to establish the 

 limits of the pulses. 



The number of forms of diatoms noted in our records in the 

 plankton of the Illinois River is thirty-one. This number could be 

 greatly increased by the inclusion of the many adventitious species 

 which flood-waters bring into the plankton and by the addition of 

 rarer limnetic species. Of these thirty-one at least twelve are 

 eulimnetic, while the others are in the main adventitious. There 

 are no species among them peculiar to the potamoplankton, and 

 the dominant forms here are also abundant in the fresh-water plank- 

 ton of our own Great Lakes and of European streams and lakes, 

 barring a few mooted points of specific identity. 



The limnetic species are fourteen in number, viz. : Asterionella 

 formosa, A. gracillima, Cydotella kuetzingiana, Diatoma elongatum 

 var. tenue, Fragilaria crotonensis, F. virescens, Melosira . granulata 

 var. spinosa, M. variant, Meridian circulare, Rhizosolenia eriensis, 

 Stephanodiscus magaroz, Synedra acus, S. acus var. delicatissima, 

 and Tabellaria fene strata. Of these limnetic forms the more impor- 

 tant ones are Asterionella gracillima, Cydotella, Fragilaria virescens, 

 Melosira granulata var. spinosa, and Synedra acus and its varieties. 

 The absence or small number of certain limnetic species is notice- 

 able. These are several species of Tabellaria and Attheya. On ac- 

 count of the abundance of silt and the transparency of Attheya it 

 may have been overlooked. It has hitherto been reported from 

 waters much nearer the sea, and this coupled with its affinities to 

 marine diatoms may explain its absence in our waters. 



The remainder of the forms are adventitious, or largely so, and 

 with the exception of the species of Navicula they have little effect 

 upon the ecology or quantity of the potamoplankton. 



DISCUSSION OF SPECIES OF BACILLARIACE^I. 



Asterionella formosa Hassall. Average number of individual 

 cells, 960. Average size of colony, 4.8 cells. Recorded only in 

 November, December, and from February through April, and never 

 in large numbers. The greatest pulse attained at any time cul- 

 minated on March 30, 1896, at 54,540. Aside from an isolated 

 occurrence on June 27, 1896, no individuals were recorded at tem- 

 peratures above 48, and three fourths of the occurrences are at 



