279 
ever, that in all tests extending over many days other factors 
than variation in local distribution come in to modify the re- 
sults. 
LONGITUDINAL DISTRIBUTION FROM THE MOUTH TO HENNEPIN. 
By courtesy of the Illinois State Fish Commission I made 
on May 18-21, 1899, a trip on their steamer “Reindeer” from 
the mouth of the Illinois to Hennepin, about 205 miles from 
the mouth, making ichthyological collections for the State 
Survey. Incidentally plankton collections were also taken 
continuously from a short distance above the mouth to Henne- 
pin—in all, 21 collections. Of these, 19 will be utilized in the 
following comparison, the first being omitted because of uncer- 
tainty as to the distance, and one other because of loss of the 
collection. The catch was made by means of a 14 in. iron pipe 
carried from the guards of the boat to a depth of 18 in. below 
the surface of the water. The intake was redueed to ? in. and 
turned toward the prow of the vessel, so that, while moving, a 
continuous stream of water was discharged into the plankton 
net, immersed in a barrel on deck. In this fashion a con- 
tinuous stream from the level of the intake was filtered. The 
contents of the net were removed approximately every ten 
miles of transit, and its clogging to the point of resistance pre- 
vented by shaking it down whenever necessary, thus minimiz- 
ing, in part at least, this source of error. The following table 
gives the data concerning these catches and the measurements 
and silt estimations,* together with my computations of the de- 
partures of the total catches from their mean and of the esti- 
mated planktons. 
The distances between points of collection were not deter- 
mined with great accuracy, since we had no log,and maps give 
no clue to the not infrequently tortuous steamboat channels. 
The distances are therefore approximations based on the expe- 
rience of the pilot and engineer in charge of the boat. 
*By the generous permission of Professor Forbes, centrifuge measurements and 
silt estimations have been kindly furnished tome by Mr. R. E. Richardson, who is 
preparing for publication in this Bulletin « detailed report upon these collections. 
