panying key sheets, each number on each sheet standing for one of 

 the more common species. One sheet was assigned to algae, 

 diatoms, and miscellaneous organisms; and one each to Protozoa, 

 Rotifer a, and Entomostraca. As the plankton sample was examined 

 under the microscope the identifications were called off, and entered 

 on the sheets by a clerical assistant. Six of the most abundant 

 species were recorded by the observer himself on six tallying 

 machines registering 1,000, and conveniently arranged in a box at 

 his right. By adjusting the springs to give different sounds when 

 registry was made, and by modifying the surfaces pressed by the 

 fingers so as to differentiate the several machines without looking 

 at them, it w r as possible to use these without raising the eye from 

 the microscope, and thus to avoid the fatigue arising from the 

 repeated muscular readjustment of the eyes necessary when the 

 observer makes his own entries in a written record. Common 

 species not recorded by the tallying machines were generally abbre- 

 viated or designated by easily-called tokens. When once fairly 

 familiar with the species it was possible by means of these labor- 

 saving devices to make identifications and enumerations of several 

 heavy planktons per day. 



By a number of tests I found that when the enumerations of a 

 species in a given collection reached 1,000, little was gained by 

 carrying it to higher numbers. A limit of error of 5 per cent. 

 can be thus obtained if the species in question is distributed evenly 

 in the cell and all precautions are observed to secure accuracy. 

 Enumerations were often carried beyond this point, but rarely beyond 

 3,000. The accessions numbers of the collections from our catalog 

 of collections served to designate each sheet of data and all note 

 slips bearing on the collection or its constituent organisms. When 

 the enumeration was completed, the factors of collection, dilution, 

 and enumeration were entered on the sheets, and the number of 

 individuals of all species represented was computed and carried to 

 the right of the sheet. The totals of the various groups for ex- 

 ample, diatoms or Cladocera were then added up and entered on 

 the sheets in differential colors. By the use of the key sheets the 

 number perm. 3 of water of any given species could be quickly ascer- 

 tained. Species not in the key were entered by name on the sheets. 



When the enumeration of all collections was completed, the 

 numbers per m. 3 giving the seasonal distribution of the various 



