183 



and aquatic, minute bits of leaves, stems, bark, and wood, with 

 the characteristic grain refuse from distilleries and glucose- 

 works and the offal from the cattle-yards at Pekin and Peoria. 

 At all seasons of the year and in all waters the scales of Lepi- 

 doptera and the pollen of coniferous trees are of common oc- 

 currence. Mingled with this material, especially when aquatic 

 vegetation is present, is a very light flocculent material con- 

 sisting, in part at least, of the zoOglceae of bacteria. It is in the 

 midst of debris of this varied composition that the plankton 

 lives, and it is in collections consisting to a greater or less ex- 

 tent of silt material that the river plankton must be studied, 

 its species determined, and its individuals enumerated. 



In collections made with the silk net the greater part of 

 the fine silt passes through the meshes with the water. In 

 filter-paper catches some of it adheres to the paper, and the 

 finer flood silts will even pass through hard-pressed filter paper 

 in small quantities. With silt of so varied a character it is 

 practically impossible to establish and continue any standard 

 of measurement or estimate which affords a satisfactory basis 

 for the determination of the relative amounts of silt and plank- 

 ton present in the collections. After considerable experience 

 in the examination of our collections I have endeavored to 

 estimate the amount of silt present in them as they appear 

 in the Rafter counting-cell. The distribution of the material 

 in the cell and the conditions of examination are such 

 as to favor a uniform standard of estimation. On the other 

 hand, the estimates are purely personal, without any volumet- 

 ric check, and are thus only comparable with each other. This 

 method seems to be the only solution at present available for 

 this perplexing problem. These estimates are given in Tables 

 III.-IX., together with computations, based thereon, of the 

 amount of both plankton and silt per cubic meter. These 

 figures form the basis of the diagrams in Plates VIII.-XIII. and 

 XXII.-XLII. As will be seen in the tables, the per cent, of silt 

 varies from a mere trace to almost the entire catch, changing 

 with the river conditions as previously, stated. 



