266 



stations included in this discussion number in all 643, distrib- 

 uted as follows: Illinois River 235, Spoon River 36, Quiver 

 Lake 115, Dogfish Lake 48, F\^g Lake 44, Thompson's Lake 99, 

 and Phelps Lake 67. 



ILLINOIS RIVER CHANNEL, STATION E. 

 (Table III.; PI. I., V., VII.— XIII.) 



DESCRIPTION OF LOCALITY OF COLLECTION. 



The collections were made two and a quarter miles above 

 the city of Havana, a short distance above the outlet of Quiver 

 Lake (PI. IL), at a point where the river w^as about 500 feet 

 in width at low water and about 600 feet from crest to 

 crest of the banks, which are here fringed by willows [Salix 

 nigra and S. longifoVia) on both sides. The eastern shore is a nar- 

 row spit, 6 to 8 feet above low water, separating the river from 

 Quiver Lake. The western bank is higher, 8 to 10 feet, and is 

 covered by bottom-land forest. This is also a spit or "towhead" 

 between the river and Seeb's Lake. At low water (PI. IV.) the 

 eastern bank is exposed as a gentle declivity of 25 to 40 feet, 

 while the western one is much wider — a belt, 50 to 75 feet in 

 width, of soft black mud with gaping cracks (PI. V.). A short 

 distance from the low-water shore-line the bank shelves some- 

 what abruptly to the bottom, which with the exception of a 

 slight ridge near the center of the channel extends in an un- 

 broken level from side to side of the stream. The depth at low 

 water for a width of over 400 feet is 8 to 9 feet. To the north- 

 ward the river deepens slightly, while towards the mouth 

 of Spoon River it shoals to 6 feet, and below it to less 

 than 5 feet. The banks are of black alluvium, hardened 

 in the upper levels by exposure at low water, but al- 

 ways soft and treacherous near the low-water line. The bot- 

 tom in the channel is firm, being a compact bed of heavy blu- 

 ish mud mingled with sand and the shells of Unionida', which 

 form in many places continuous beds of large area. 



A slight curve in the river above our plankton station shifts 

 the current at that point towards the eastern shore, but at the 



