342 
catches of the silk net is to some extent misleading, owing to 
the relatively greater proportion which the escaping planktonts 
form of the production in the tributary stream. Another factor 
which prevents an equally accurate volumetric determination 
of the plankton of the two streams is the presence in Spoon 
River of a much greater proportion of silt. For example, in 1896 
and 1898 the estimated ratios of silt and plankton in the average 
of the catches (Table IV.) is .007 to .349 and .029 to .796. In 1897, 
when low water and slight current and some probable invasion 
of channel waters increased the plankton production, the ratios 
are 1.257 to 1.173. The ratios of the first-named years are more 
nearly normal for this tributary, and in such ratios it is quite 
probable that the error in silt estimation to some undetermined 
degree tends to prevent any precise determination of the actual 
plankton production. Nevertheless, after a very wide margin 
is allowed for probable error in the data, the comparison of 
production in the two streams is instructive and significant, for 
it is the direction of change or contrast in production which is 
of greatest consequence, and this may be found even in the 
presence of a large but distributed error. 
It should be noted that the plankton ordinants in the 
Spoon River plates (XXII.—XXIV.) are plotted on a scale ten 
times that of all other stations in order to give an appreciable 
height to the plankton portion of the entry. 
PLANKTON PRODUCTION. 
1896. 
(Table 1V.; Pl. XXII., XLVI.) 
For purposes of comparison I introduce at this point a 
table which gives in terms of monthly averages of plankton 
in em.’ per m.’ the relative production in the seven locali- 
ties examined by us. The number of collections entering 
into each average is stated, and the grand average of all collec- 
tions and of the monthly averages are given for each station. 
In 1896, nine collections were made in Spoon River in 
August-December, the average being only .007 cm.’ per m.° 
