110 



The location of the pulses of D. globulosa bears no constant rela- 

 tion to those of other organisms, owing, in part, at least, to the 

 irregularities of the floods upon which some of them seem to depend. 

 The great pulse of Sept. 7, 1897, is intercalated between two pulses 

 of diatoms and other chlorophyll-bearing organisms, and some 

 others bear a similar relation to their food supply, while some co- 

 incide with an increase in these synthetic organisms (cf. Table I. 

 and PL II.). 



Difflugia globulosa and the following species were reported by 

 Smith ('94) in the plankton of Lake St. Clair; by Jennings ('OOa) in 

 that of Lake Erie; and were common in the plankton of Lake 

 Michigan (Kofoid '95). Difflugia of the forms included here under 

 D. globulosa and D. lobostoma have been reported by many authors 

 from various European lakes and rivers, but in no reported instance 

 do they reach the numbers or importance in the plankton that they 

 do in the Illinois. Full records of their seasonal distribution may, 

 however, bring such importance to light. 



Difflugia lobostoma Leidy. Average number, 1,158. In the 

 total of all collections it is about one fifth as abundant as D. globu- 

 losa. Like that species it occurs throughout the whole year in 

 almost every collection (Table I.), and the fluctuations in its occur- 

 rence follow very closely those just described for D. globulosa in the 

 direction of their movement. The amplitude of the pulses is less, as 

 a rule, and their culminations and limits are coincident, or at least 

 approximate. Thus, on Sept. 7, 1897, D. lobostoma attains only 

 24,000, and the pulse of D. globulosa on June 28 (80,000) is attended 

 by one of 96,000 in D. lobostoma in the next collection, on July 14. 

 There are in this species also the same influx into the plankton 

 with floods, and increase in numbers at temperatures above 60. 

 There are 954 per collection per cubic meter below this temperature 

 to 1,436 during the warmer months in 1898. There are also pulses 

 during the warmer months, in stable conditions, coincident with 

 those of D. globulosa. Similar causes presumably contribute to 

 these results in both species. 



Difflugia lobostoma is also exceedingly variable in proportions, in 

 the texture of the shell and the degree of incision, and in the num- 

 ber of lobes about the mouth. Two, three, and even four have been 

 noted, and they vary greatly in depth, in regularity, in perfection 

 of their development, and in the structural border which sometimes 



