6 
panying key sheets, each number on each sheet standing for one of 
the more common species. One sheet was assigned to alge, 
diatoms, and miscellaneous organisms; and one each to Protozoa, 
Rotifera, 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 was 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—tor 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 per m.° 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.* giving the seasonal distribution of the various 
