22 



gests the action of this factor, and if this be true, it becomes a tem- 

 porary rather than an adventitious planktont. Temperatures seem 

 to bear little relation to the occurrence of Oscillatoria in the plankton. 



Tetrapedia emarginata Schrod.* Average number, 242,308. 

 From the first of August till the end of October in numbers from 

 1,000,000 to 3,500,000 per m. 3 , appearing later and in larger num- 

 bers in October in 1897 than in 1898. At temperatures above 65. 



Tetrapedia gothica Reinsch, Glceocapsa polydermatica Kutz., and 

 Gloeocapsa sp. were recorded once or twice in the midsummer plank- 

 ton in relatively small numbers. 



CHLOROPHYCEyE. 

 (Plates I. and II.) 



Average number, 53,175,105, including, without duplication, 

 species from both silk and filter-paper collections. In 1897 this 

 was very much greater (139,739,850), owing to the prolonged low 

 water and higher temperatures of the late autumn. Although 

 abundant, these organisms are outnumbered by the diatoms six to 

 one, and by the synthetic Mastigophora by about two to one. The 

 ChlorophycecB of the plankton, with few exceptions, are minute, and 

 generally escape through the silk net. Pediastrum and colonies of 

 Botryococcus are about the only species of which the usual method 

 of plankton collection in our waters affords a fair representation. 



The Chlorophycea appear in every collection examined through- 

 out all the years of our operations, with the exception of eight in 

 midwinter floods in 1895 and 1896. As a group they are adapted to 

 the whole range of temperatures, and exhibit in 1897, on April 28, 

 a well-defined vernal pulse of 367,200,000, and a series of autumnal 

 pulses culminating September 21 at 216,000,000, October 19 at 

 367,200,200, and November 23 at 52,000,000. In this year the 

 -midsummer pulses are of minor importance in comparison with 

 those of spring and autumn. In 1898 the vernal pulse is also well 

 defined, culminating May 3 at 212,406,400, and it is followed by a 

 series of four midsummer pulses of considerable magnitude, w r hich 

 culminate June 14 at 46,000,000, July 19 at 277,000,000, August 9 

 at 370,000,000, and August 30 at 189,000,000. The autumnal pulse 

 appears September 27, attaining 70,526,400. The summer and 

 autumn hydrographs of this year are much more disturbed than in 



