34 
It is apparently a phenomenon of seasonal acclimatization, by virtue 
of which the low temperatures of the winter lower the optimum for 
the vernal pulse, and the high temperatures of the summer raise 
it for the autumnal pulse. 
Selenastrum bibraianum Reinsch.*—Average number 519,235. 
Recorded only from the beginning of August till the end of Novem- 
ber, and never in great abundance. Slight evidence of a September 
pulse. 
Some other Chlorophycee have been included in the totals as 
“unidentified,” and isolated occurrences of the following have been 
noted: Cerasterias longispina (Perty) Reinsch, C. raphidioides 
Reinsch, Dactylococcus imjfusionum Nag., Gleocystis gigas (Kiitz.) 
Lagerh., Staurogenia lauterbornt Schmidle, and a few of the Con- 
fervacee—which are probably adventitious. These are a species of 
Conferva, of Prastola, and of Ulothrix—all of which appear sparingly 
in spring and autumn planktons, the first-named and the last 
as minute filaments in the filter-paper collections. A thorough 
analysis of the unidentified forms would greatly extend the list of 
species and varieties. 

BACILLARIACEA. 
(Plates I. and II.) 
Average number, 396,192,716, including, without duplication, 
diatoms from both silk and filter-paper collections. They were 
almost twice as abundant in the more stable conditions 1n which the 
collections of 1897 were made. The Bacillartacee are more abun- 
dant than any other synthetic group of organisms in our plankton. 
They exceed (in 1898) the Schizophycee five to one, the Chloro- 
phycee seven to one, the desmids eight thousand to one, and the 
synthetic Mastigophora by more than four to one. Their numerical 
preponderance 1s, with the exception of the synthetic Mastigophora, 
equaled or exceeded by their relative quantitative significance in the 
ecology of the plankton. 
They appear without exception in every collection, and their 
seasonal distribution in its main features is repeated from year to 
year. There is a principal vernal pulse in April-May and a hiemal 
pulse in November—December. Minimum periods separate these 
pulses and are varied by other pulses, usually of minor importance, 
at intervals, in 1898, of three to five weeks. The winter minimum 
