18 Illinois State Laboratory of Natural History. 



catalogue number, designation of the catch (whether surface, 

 bottom, vertical, or qualitative), station, killing agent, and 

 date. For convenience in handling they are then arranged 

 chronologically in racks, each holding six bottles. Data blanks 

 bearing the catalogue number are rilled out for each station 

 examination. The locality, date, time of day, the condition of 

 the sky, the direction and force of the wind, the stage of the river 

 and the amount and direction of its change in the twenty-four 

 hours preceding, the depth of the water, its turbidity (measured 

 by means of a porcelain disk), the disturbance of the surface, 

 the temperature of the air and that of the water at surface 

 and bottom, the current, the kind of vegetation and distance 

 from it, the manner of collection and means of preservation 

 of the catches made, — all are matters of regular record, 

 together with any other data peculiar to the collection which 

 could possibly interest the student of the plankton. 



2. Quantitative Examination. — The quantitative exam- 

 ination of all of the plankton collections made at Havana has 

 been undertaken by the writer. Determination of the quan- 

 tity of the plankton by both the volumetric and enumeratiye 

 methods is necessary, owing to the presence, especially in 

 flood waters, of a large amount of silt. The gravimetric or 

 weighing method suggested by Zacharias ('95) is, as Ward 

 ('96) has suggested, objectionable on account of the unknown 

 and presumably variable amount of water or alcohol present 

 in the still moist plankton. Many of the planktons at 

 Havana taken in silt-laden waters contain a considerable 

 amount of mineral and earthy matter. This constitutes a 

 further objection to the application of the gravimetric method 

 to our collections. A combination of the gravimetric and 

 volumetric method has been suggested by Ward ('96) ), in which 

 plankton of known volume is dried to a constant weight, burned, 

 the ash weighed and afterwards digested in concentrated 

 HC1, and the residue then washed, dried, and weighed. The 

 amounts of organic matter, of soluble salts (calcareous), and 

 of silicious matter can then be determined, and thus correc- 

 tions for sand-laden planktons can lie approximately com- 

 puted. It is evident that this method cannot be applied to 



