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 H 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 reduced 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 a detailed report upon these collections. 



