66 
intervals of several weeks, and that such pulses can also be traced 
back into 1897 as far as the collections were made at weekly inter- 
vals—that is to the early part of July. A similar periodicity on the 
part of the Mastigophora—the greater part of which are also 
chlorophyll-bearing—is even more evident. Not only is this 
periodicity present in this group, but it coincides approximately in 
the location of its maxima and in their relative development with 
that found in the Chlorophycee and Bacillariacee. The following 
table, which gives the dates of culmination of the pulses of these three 
groups from July 1, 1897, to April 1, 1899, will serve to demonstrate 
this point more clearly, and a graphic presentation of the data will 
be found in Plates I. and II. 
There are twenty-two of these recurrent pulses in the period 
from July, 1897, to March, 1899. Of the sixty-six possible maxima 
only five are missing,or at least not apparent in our data, and but 
ten culminate on other dates than the one (of collection) most to be 
expected. These ten in every case culminate either a week prior or 
subsequent to that in which the other two groups reach their max- 
ima. These divergences may be due to the error incident to the 
interval of collection, and their approximation in time is still cor- 
roborative of the tendency towards recurrent periods of growth. 
These exceptions are no greater than might be expected to occur in 
the unstable fluviatile environment and within the large margin of 
error of the plankton method. 
There are twenty-one intervals between July 14, 1897, and 
March 14, 1899, with a range in length of 20 to 42 days and an aver- 
age of 28.95. The intervals in days with the numbers of instances 
of each are as follows:. 20° (1); 28 G);- 2250), .23.()) 2 Zone 
27 (1), 28 G7), 35 @), and 42 (3), days.” Uhevetiect of theaweelay: 
interval of collection is seen in the preponderances at 21, 28, 35, and 
perhaps at 42, days. There is evidently a tendency towards the 
interval of 28 days. Nine of the 21 pulses are grouped about this 
interval: 6, about that of 21: while 3 are at 35 and 3 at 42. It there 
be such a tendency itis but natural that with a weekly interval of 
collection there should also appear minor preponderances at 21 and 
35 days. Traces of a similar rhythm may be found in the period of 
weekly collections in 1896 (Pt. I., Table IIT.). 
In some instances the environmental conditions at these times 
of departure are such as to suggest that they may have produced the 
