102 
average number of individuals in these cases was, however, only 
230 per cubic meter as against 1,443 when the temperature was 
above 55°, or, if below, when floods prevailed. The seasonal and 
numerical distribution of occurrences and individuals alike point 
to the agency of floods and higher temperatures in the introduction 
of Arcella into the plankton from its usual habitat, the bottom and 
the shore. 
This species occurred in water ranging in temperature from 32° 
to 89°. Being a bottom form, the plankton data. do not afford a 
satisfactory basis for determining its true seasonal distribution and 
optimum temperature. The maximum number found, 25,272, was 
in water at 32°; but this was an isolated occurrence in a flood, and 
serves only to illustrate the irregularity of distribution in the 
plankton of tycholimnetic organisms. 
Centropyxis aculeata Stein.—Average number, 570. This species 
has appeared in collections in every month of the year, but its 
sequence is frequently interrupted and its numbers are quite irregu- 
lar. Practically without exception all the larger occurrences attend 
rising flood waters. It is evidently adventitious at all seasons of 
the year. 
Centropyxis aculeata var. ecorms (Ehrbg.) Leidy.—Average 
number, 604. In former years this species was less frequent than 
the preceding species. Its appearances in the plankton tend to 
coincide with those of C. aculeata (Table I.), and are doubtless due 
to the same causes. Thus in the February flood of 1898 there is a 
pulse of 12,636 of C. aculeata and one of 9,477 of var. ecornis. 
C. levigata Penard seems to be identical with this variety. The 
data concerning both C. aculeata and its variety ecornis are too 
irregular to throw any light on the seasonal cycle of these adventi- 
tious planktonts. 
Cochliopodium bilimbosum (Auerbach) Leidy.—Average number, 
1,384. This species was found in the plankton during 1898 in 
irregular numbers in 27 of the 52 collections. The distribution of 
the occurrences affords indubitable proof of their close dependence 
upon flood waters. In 15 of the 27 cases Cochliopodium appeared 
with a rising river, and in all but 6 cases, in periods of considerable ~ 
movement in river levels (cf. Table I. with Pl. XII., Pt. I.), such 
as the rising flood of January and February and the repeated minor 
