550 
culent matter occupies a large part of the field as the catch is 
seen in the Rafter cell. It is a difficult matter to estimate 
with any certainty the proportion which such silt forms of the 
filter-paper catches. Nor is it possible to use the same standard 
of estimation upon the catches of both the silk and filter-paper, 
owing largely to the considerable volume of the flocculent debris 
in the distributed plankton and the uncertainty as to itscompres- 
sibility in the centrifuge. My estimates of the silt, which will 
be found in the table beginning on page 552, in the light of the 
tabulated results of the enumeration seem to be too large, 
probably as the result of the influence of the standard used on 
the silk catches. It is, therefore, my opinion that the quanti- 
ties of plankton given in this table are in the main below 
rather than above the actual amounts present. The facts upon 
which this opinion is based will appear in the discussion of the 
results of the enumeration. It should be said that the relative 
values of the estimated quantities of plankton in this table are 
probably above the actual ones. They are of more importance 
in indicating the direction of the seasonal movement in pro- 
duction than they are in expressing the exact amounts of 
plankton present. 
Because of this great increase in the proportion and quan- 
tity of the silt found in these filter-paper catches, even in our 
clearest waters, the filter-paper method does not satisfactorily 
solve the volumetric problem. It is not a satisfactory method, 
and, indeed, it seems probable that the same difficulty will be 
met in all methods which remove all suspended solids in all 
but the clearest lake or ocean waters. This same difficulty has 
balked the efforts of Lohman (’03) to determine satisfactorily 
the volumetric loss by leakage through the silk in the applica- 
tion of the Hensen method to a marine plankton, which leaks 
through the silk. In passing, it may be noted that this plank- 
tologist of the Hensen school at Kiel, working with the subven- 
tion of the University of Kiel and the German Commission for 
the Investigation of the Sea, after testing upon the plankton of 
the Baltic and Mediterranean seas the correctness of my criti- 
