60 



in this condition they have often been taken in some quantity in 

 the plankton of the river, but they are so plainly adventitious and 

 irregular that no notice has been taken of them in our enumeration 

 work, and when possible they have been removed before measure- 

 ment or deducted by estimation from the volumetric records. 



The desmids are few both in species and individuals. Seven 

 species have been recognized, of which but four are of general 

 occurrence in the plankton. These are three species of Closterium 

 and Staurastrum gracile. The latter and Cosmodadium saoconicum 

 are the only eulimnetic organisms among them. The center of dis- 

 tribution of the other species is the shore and bottom. The stom- 

 achs of fish such as the CatostomidcB, the carp, and Dorosoma cepe- 

 dianum, which often feed upon the bottom ooze or slime about 

 aquatic plants, usually contain many desmids, including the species 

 here noted. Other species also are occasionally adventitious in the 

 plankton, and the list might be considerably extended, though the 

 absence of extensive peat bogs in the drainage basin of the river 

 reduces the desmids to a position of much less importance than that 

 which they occupy in more northerly waters. 



As a group they exhibit a well-defined seasonal distribution, 

 with a vernal pulse at about the time of the volumetric maximum 

 in April-May and an autumnal pulse of less regular occurrence, 

 location, and size. The optimum temperature for their appear- 

 ance in the plankton lies below 70, and in winter months they 

 occur but rarely. 



DISCUSSION OF SPECIES OF CONJUGATE. 



Closterium acerosum Ehrbg. Average number, 348. More than 

 three times as abundant in the previous year. This desmid is 

 perennial in the plankton, having been found in every month of the 

 year, but at irregular intervals, and never in large numbers. Its 

 distribution is such as to suggest that it is at the most only semi- 

 limnetic in habit. The numbers are too small to follow closely the 

 seasonal distribution. There are pulses on May 3 (3,200), Septem- 

 ber 6 (2,400), and November 1 (2,500) in 1898; and in 1897 a pulse 

 on June 28 (2,000) and one on September 21 (24,000). In previ- 

 ous years vernal pulses in April and occasional autumnal pulses are 

 to be noted. In so far as the optimum temperature is indicated, it 



