eS 
241 
where its numbers are greater. The forms known as apicata 
Kurz and kahlbergiensts Schoed. appear in small numbers in some 
years. . 
This species appears in our collections in April-December only, 
with the exception of one occurrence in January and two in March. 
Its occurrences and numbers vary greatly in different years. In 
1894—95 its numbers were small and occurrences scattering, it being 
~ most abundant in November—December. In 1896 there was a 
large vernal development in April-June, and a series of diminishing 
pulses in July-September. In 1897 no vernal development appeared 
in our scattered collections, but in the stable conditions of late 
summer and autumn occurred the largest development recorded in 
any year, with a maximum record of 72,760 per m.?on October 5. In 
1898 there was a small vernal development (3,400) in May—June 
and a still smaller one (600) in October. A well-defined seasonal 
routine is thus not demonstrable from our data, though the fact 
that both the percentage of occurrences and the numbers are highest 
in May—June and September—October suggests a tendency toward 
vernal and autumnal pulses separated by a period of less develop- 
ment in midsummer and of autumnal decline followed by a period 
of almost complete extinction in midwinter. 
The statistics of the D. cucullata population in all years in which 
weekly collections were made, exhibit very clearly the phenomenon 
of recurrent pulses of 3 to 5 weeks’ duration, with maxima of varying 
amplitude and minima of less than 400 per m.? in all cases but those 
which mark the September pulse of 1897. There are in 1896 pulses 
culminating April 24 (2,544 per m.’), May 8 (11,965), June 11 
(12,000), July 18 (1,040), August 8 (800), and September 16 (507). 
In 1897, vernal records are incomplete. Pulses appear July 14 (800), 
August 17 (1,680), September 14 (57,000), October 5 (72,760), and 
November 15 (2,040). These pulses coincide exactly or approxi- 
mately with those of the other Entomostraca which exhibit the same 
phenomenon, and approximate also those of the Rotifera. A typical 
pulse, that of October, 1897, is shown in the following table. It 
is a noticeable fact that the proportion of immature forms is often 
greater at and after the period of maximum development than at 
other times, as appears in the table. 
The relations of temperature to the development of D. cucullata 
in channel waters appear in the fact that all occurrences in excess of 
