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,-1s 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: Ply I): 
Difflugia globulosa and the following species were reported by 
Smith (94) in the plankton of Lake St. Clair; by Jennings (’00a) in 
that of Lake Erie; and were common in the plankton of Lake 
Michigan (Kofoid ’95). Dzfflugza 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 
