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 Chlorophycecs 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 infusionum Nag., Glceocystis gigas (Kiitz.) 

 Lagerh., Staurogenia lauterborni Schmidle, and a few of the Con- 

 fervacecz which are probably adventitious. These are a species of 

 Conferva, of Prasiola, 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. 



BACILLARIACE^E. 

 (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 in which the 

 collections of 1897 were made. The Bacillariacece are more abun- 

 dant than any other synthetic group of organisms in our plankton. 

 They exceed (in 1898) the Schizophycecz five to one, the Chloro- 

 phycecs seven to one, the desmids eight thousand to one, and the 

 synthetic Mastigophora by more than four to one. Their numerical 

 preponderance is, 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 



