58 
Stephanodiscus niagare Ehrbg., a common planktont in the 
waters of the Great Lakes, appeared but once, in May, in our plank- 
ton, though the river had for years received, by way of the Chicago 
River, constant access of water from Lake Michigan. The turbid, 
sewage-laden, and warmer waters of the Illinois are evidently not 
favorable for its growth. 
Surirella ovalis Kitz. var. minuta (Bréb.) Kirchner.*—Average 
number, 761,538. Present sparingly throughout the year, bap 
principally during summer months. Vernal pulse in May. 
Surtrella spiralis Kiutz.—Average number, 1,612. Less abundant 
in the more stable conditions of 1897. This species is most abun- 
dant in Quiver Creek and Spoon River. Its fluctuations are slight, 
irregular, and often appear with flood waters, all of which phenom- 
ena indicate its adventitious character in the river plankton. 
Synedra acus Kiitz.*— Average number, 36,558,462 (silk, 308,- 
330). This species is a perennial planktont, appearing, for example, 
in 1898 in every collection. It has a highly developed and shifting 
vernal pulse, and an inconstant and but slightly developed autumnal 
or hiemal pulse. The vernal pulse appears in 1895 on April 9 at 
209,880; in 1896 on April 24 at 366,828; in 1897 on May 25 at 
2,620,800 (82,800,000*); and in 1898 on May 10 at 9,043,200 
(813,600,000*). The second pulse appears in 1895 on November 
14 at 99,360; in 1896 on December 3 at 44,464; in 1897 no pulse 
occurs; in 1898 it occurs on November 8 at 19,000. As in some 
other diatoms, there are minor pulses throughout the year, though 
in this case they are all feebly developed, exceeding 100,000 (silk) 
in but a single instance. The minor pulses of midwinter often 
exceed in prominence those of midsummer. The meteoric char- 
acter of the vernal pulse is very pronounced in this species both in 
the suddenness of 1ts appearance and its disappearance and in the 
height which it attains. 
The variety delicatissima W. Sm. is included here with the type 
acus. During the autumn of 1898 a separate record was kept of 
the two, with the result that the variety appears to include about 
four fifths of the individuals at that season. The two are not 
readily separated. The colorless form recently described by Pro- 
wazek (’00) as S. hyalina is also included, and it is not uncommon 
when S. acus is abundant. Colorless forms of other diatoms of the 
plankton, as Asterionella, Melosira, and Fragilaria, also occur, but 
