58 



Stephanodiscus niagarce 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 Kiitz. var. minuta (Breb.) Kirchner.* Average 

 number, 761,538. Present sparingly throughout the year, but 

 principally during summer months. Vernal pulse in May. 



Surirella spiralis Kiitz. 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 its 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 tw T o are not 

 readily separated. The colorless form recently described by Pro- 

 wazek ('00) as 5. 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 



