265 
assigning an empirical coefficient to each catch. This coeffi- 
cient was decided upon after analysis or inspection of the plank- 
ton, and in view of its quantity and constituent organisms, the 
amount and nature of the silt, and the age of the net, the basis 
of estimation in each case being the coefficient test by the 
pumping method whose conditions most nearly approached 
those of the catch in question. These coefficients were decided 
upon without knowledge of or reference to the effect which 
they might have upon the theoretical questions arising from 
the analysis of the quantitative results, and prior to the organ- 
ization and analysis of the volumetric data. 
Obviously this latter method involves both possible and 
probable error in estimation of similarities and differences in the 
catches examined and in maintaining throughout a uniform 
standard. Nevertheless, for plankton catches as varied as those 
with which we deal, it is probable that this method involves less 
distortion of volumetric results than the omission of the coeffi- 
cient factor or the adoption of a uniform factor for all catches 
irrespective of the fluctuations in this factor as revealed by 
our field tests. Accordingly all of the volumes of catches by the 
drawn net, of plankton, silt, and total catch per cubic meter 
recorded prior to May 20, 1896,in Tables III.-IX. have been 
computed with this coefficient as one of the factors, the actual 
factor employed being given in the tables in each case. 
The results of my efforts (see Kofoid, 97a) to find an ade- 
quate correction for the loss by leakage through the silk by the 
use of hard-pressed filter paper and the Berkefeld army filter 
will be discussed in another connection. 
The volumetric data of the plankton at the seven stations 
(see Pl. II.) at which periodical collections were made, name- 
ly, the river channel (E), Spoon River (M), Quiver Lake (C), 
Dogfish Lake (L), Flag Lake (K), Thompson’s Lake (G@), and 
Phelps Lake (F) will now be discussed, and the general ques- 
tions arising from the investigation as a whole will then be 
treated. 
The chronological series of collections at these seven 
