537 



in the main stream. They are essentially littoral forms es- 

 pecially common about driftwood, which abounds in Spoon 

 River — a fact which doubtless accounts for their frequent oc- 

 currence in the plankton of that stream. 



The total number of species of planktonts noted in the col- 

 lections from the two streams in my enumerations of the 

 plankton, and listed in Table XIV., is not to be taken as repre- 

 senting the total number present. But a small part of each 

 catch was examined, and rare species in both streams usually 

 escaped detection. Their relation to each other, however, af- 

 fords an index of the relative number of species present in dif- 

 ferent collections in the same or different streams provided the 

 method of examination is similar. The silt makes uniformity 

 of dilution impossible in many cases, and thus introduces some 

 error into the data. The average number of species noted in 

 a Spoon River collection was 24 to 69 in the Illinois, a differ- 

 ence .less marked than that disclosed by the volumetric or the 

 statistical comparison, but still significant of the relative 

 paucity of the plankton of Spoon River and the more limited 

 range of its constituent organisms. 



The total number of species recorded from Spoon River is 

 170, while the Illinois yielded 429. The greater number of col- 

 lections examined from the latter stream may explain a part 

 of this difference. Without exception, all the identified species 

 found in Spoon River have occurred also in the Illinois above 

 the mouth of Spoon River. The only species in the tributary 

 which seem to reach as great a development there as they do in 

 the Illinois, or a still greater one, are the species of Surirella, 

 especially 8. splendida, several forms of Closterium, Cyclops 

 serrulatus, and the dipterous larvae above mentioned. 



A comparison of the total population of the two streams 

 throughout the thirty months covered by our collections, as 

 shown in the final columns of Table XIV., throws some further 

 light on the relations between the plankton of the two-streams. 

 Traces of the same seasonal routine appear in both ; the spring 

 maximum appears in both at about the same time, while the 



