214 
PuLSES OF SYNCHATA STYLATA—continued. 
Year Date Temp. No. Date Temp. | No. Date Temp. No. 
1894 | 
1895 | Aug. 1 79° 10,287 Sept. 27 ee 12,225 
1896 — == || Ane, & 86° 2400). || ———— — 
1897 July 21 ale 103,200 Sept. 7 80° 28,000 
1898 July 19 84° 64,800 Aug. 2 79° 170,400 Sept. 27 13% 265,600 
| | Pe AS) 82° 24,800 
} } 
| rs 
Year Date Temp. No. Date Temp. No. Date Temp. No. 
58° 63,935 == ——= 
1895 Nov. 27 33 901,901 | Dec. 11 32°) Ten OS6 
1896 Nov. 17 44° 4 0008 —— —— 
1897 Octia a5 “ite 12,000 | Nov. 9 SOs 26,400 | Dec. 14 36° 72,200 
Pigeon eho) 65° 15,800 om SOR GAS S 87,200 
1 898 Oct. 25 49° 824,500 | Nov. 15 41° 110,000 | Dec. 6 34° 42,500 
Sie AQ) 33° 59,200 
This is the most abundant of all the rotifers in our plankton, 
exceeding by 30 per cent. Polyarthra, the next in abundance. It 
constituted one fifth of the total Plowma in 1898, and is accordingly 
a large factor quantitatively and ecologically in the economy of the 
plankton of the Illinois River. 
It is a perennial planktont, occurring in six sevenths of our 
collections and usually in considerable numbers. The distribution 
in 1898 (Table I.) is a fair index of the usual seasonal routine, with 
the exception that in all prior years the July-August minimum is 
more pronounced and better sustained. The development in 
January-February is never large, rarely exceeding 20,000. In 
March, numbers rise rapidly, usually with a minor pulse, the re- 
covery from which in April culminates in a vernal pulse, which in 
three of the six years was the largest of the year. Following this 
vernal pulse there is a series of smaller pulses throughout the sum- 
mer. The decline of the June flood, when this occurs, seems to offer 
favorable conditions (cj. foregoing table and Pt. I., Pl. IX.—XII.) 
for the development of a pulse which is but little smaller than the 
vernal one. It may be of some significance that this pulse and the 
