50 



Diatoma vulgare Bory occurred sparingly at irregular intervals, 

 and is apparently an adventitious species in the plankton. 



Encyonema prostratum (Berk.) Ralfs appears a few times during 

 the summer months, and is evidently adventitious, as is also the 

 still rarer Epithemia turgida Kiitz. 



Fragilaria crotonensis (Edw.) Kitton. Average number of cells, 

 2.1. This limnetic diatom is much less abundant in our waters 

 than the following species. In 1898 it appeared in February, and 

 increased from 19,200 on April 19, to 14,469,120 on May 10, dis- 

 appearing entirely from the records after May 17. Such meteoric 

 pulses were not detected in previous years, when only scattered 

 entries in April, May, and December were recorded. The number 

 of cells in the filaments is very much less than in F. virescens, aver- 

 aging but 14 to its 108. Its optimum temperature lies about 60, 

 and its vernal pulse occurs immediately after the volumetric maxi- 

 mum (Pt. I., PI. XII.) and upon the same date with that of F. vi- 

 rescens. It seems to be predominantly a vernal planktont in our 

 waters. In German lakes Apstein ('96) finds maxima as late as 

 June-July, but always, it seems, at temperatures below 70. 



Fragilaria virescens Ralfs. Average number, 73.1. Apparently 

 ten times more abundant than in 1897, as a result possibly of the 

 absence of collections during the period of the vernal maximum in 

 that year. This is a perennial organism, with two well-defined 

 pulses ; a vernal one in April-May and another in November- 

 December. The uniformity with which these pulses appeared in 

 1895-1898 is very striking when one considers the unstable environ- 

 ment in which the pulses occur. In 1894 the species is not present 

 in numbers in any of the scattered collections of the year. In 1895 

 the vernal pulse is indicated in the collection of April 29 (2,754,675), 

 after which the species disappears until September, increasing 

 with a temporary backset by the December flood (Pt. I., PI. IX.) 

 to a second culmination December 30 at 282, 225. After a minimum 

 in January, 1896, the numbers increase, with minor fluctuations, to 

 a vernal maximum of 76,224,000 on April 24, followed by a mini- 

 mum period from May 18 to the following November. The winter 

 pulse again appears in December, culminating on the 3d at 867,048. 

 In 1897 the vernal pulse seems to culminate somewhat later than 

 usual, though the interval of collection is too great to follow its full 

 course. The maximum appears on May 25 at 3,549,600, after 



