183 
and aquatic, minute bits of leaves, stems, bark, and wood, with 
the characteristic grain refuse from distilleries and glucose- 
works and the offal from the cattle-yards at Pekin and Peoria. 
At all seasons of the year and in all waters the scales of Lepi- 
doptera and the pollen of coniferous trees are of common oc- 
currence. Mingled with this material, especially when aquatic 
vegetation is present, is a very light flocculent material con- 
sisting, in part at least, of the zodgloee of bacteria. It isin the 
midst of debris of this varied composition that the plankton 
lives, and it is in collections consisting to a greater or less ex- 
tent of silt material that the river plankton must be studied, 
its species determined, and its individuals enumerated. 
In collections made with the silk net the greater part of 
the fine silt passes through the meshes with the water. In 
filter-paper catches some of it adheres to the paper, and the 
finer flood silts will even pass through hard-pressed filter paper 
in small quantities. With silt of so varied a character it is 
practically impossible to establish and continue any standard 
of measurement or estimate which affords a satisfactory basis 
for the determination of the relative amounts of silt and plank- 
ton present in the collections. After considerable experience 
in the examination of our collections I have endeavored to 
estimate the amount of silt present in them as they appear 
in the Rafter counting-cell. The distribution of the material 
in the cell and the conditions of examination are such 
as to favor a uniform standard of estimation. On the other 
hand, the estimates are purely personal, without any volumet- 
ric check, and are thus only comparable with each other. This 
method seems to be the only solution at present available for 
this perplexing problem. These estimates are given in Tables 
IIl.-IX., together with computations, based thereon, of the 
amount of both plankton and silt per cubic meter. These 
figures form the basis of the diagrams in Plates VIII—XIII. and 
XXII-XLIL. As will be seen in the tables, the per cent. of silt 
varies from a mere trace to almost the entire catch, changing 
with the river conditions as previously stated. 
