241 



where its numbers are greater. The forms known as apicata 

 Kurz and kahlbergiensis 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. 3 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. 3 in all cases but those 

 which mark the September pulse of 1897. There are in 1896 pulses 

 culminating April 24 (2,544 per m. 3 ), 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 



