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 Clostertum 
and Staurastrum gracile. The latter and Cosmocladium saxonicum 
are the only eulimnetic organisms among them. The center of dis- 
tribution of the other species 1s the shore and bottom. The stom- 
achs of fish such as the Catostomide, 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 CONJUGAT#. 
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 sem1- 
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 
