86 
12. There was a pulse on July 21 of 18,400 and another on Septem- 
ber 7 of 600,000. In previous years the occurrences were scattering, 
but confined to July, August, and early September. It is evidently 
a summer planktont, whose optimum temperature lies near the max- 
imum attained by our waters. No record of occurrence below 60° 
was made. The smaller and younger colonies escape readily 
through the silk net. Its pulses in 1897 coincide very closely with 
those of Gonium, Pandorina, Eudorina, and Pleodortina. 
Pleodorina californica Shaw.—Average number, 11. In 1897 
this species, in common with other members of the family, was much 
more abundant than in any other year of our work, stable conditions 
of low water with the accompanying sewage contamination seeming 
to favor its development. The earliest record for P. californica 
in the plankton is May 18, 1896, at 71°. This was a year of lower 
water and higher temperatures than usual in spring months (Pt. L, 
Pl. X.). In other years P. californica did not appear until June or 
July. It continues into September, the latest record in 1895 being 
October 2. In 1897 there were pulses on July 21 (5,600) and 
September 7 (4,000). The occurrences at other seasons are too 
scattered to trace the seasonal fluctuations, but there is a well-de- 
fined predominance during the period of maximum heat. This is 
evidently a summer planktont, whose optimum temperature les 
near 80°. 
Pleodorina illinoisensis Kofoid.—Average number, 6,917 in 1897. 
This is somewhat more numerous than the preceding species, and 
its range of occurrences is quite similar. Its maximum pulse in 
1897 (180,000) is on August 31, a week earlier than in other members 
of the family. These pulses of the Volvocide occur (Pt. I., Pl. 
XLIV.) in a depression of nitrates and just prior to the volumetric 
pulse of September, 1897. This pulse is doubtless built up partly 
at their expense. Their decline in numbers corresponds with its 
rise. This is also a summer planktont, and was not recorded 
below 71°. 
Salpingeca brunnea Stokes.*—This species was not recorded in 
1898. Average number in 1897, 1,887,356. It occurred on May 
25 and July 21, dates of culmination of pulses of Melosira granulata 
var. spinosa. In August-September a pulse occurs, culminating 
September 7 at 47,250,000—a week after the culmination of a Mel- 
osira pulse. In 1896 (silk collections only) it was present through- 
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