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 Pleodorina. 



Pleodorina calif arnica 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. calif ornica 

 in the plankton is May 18, 1896, at 71. This was a year of low^er 

 water and higher temperatures than usual in spring months (Pt. I., 

 PI. X.). In other years P. calif ornica 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 lies 

 near 80. 



Pleodorina illinoisensis Kofoid. Average number, 6,91 7 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 VolvocidcB occur (Pt. I., PI. 

 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. 



Salping&ca 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- 



