336 



Birge — The Crustacea of the Plankton. 



The following table gives the average number of D. hyalina 

 from fall to spring. 



Table XV. — D. hyalina. Averages from October to June expressed in 



thousands per square meter. 



October 16-31.... 

 November 1-15 . . . 

 November 15-30. . 

 December 1-15 . . . 

 December 16-31 , . 



January 1-15 



January 16-31 



February 1-14 — 

 February 15-28... 



March 1-15 



March 16-31 



April 1-15 



April 16-30 



May 1-15 



May 16-31 



1894. 



252.5 

 183.1 



121.5 



(49.0) 

 40.8 



(55.9) 



(75.3) 

 65.8 

 34.7 

 63.6 

 26.4 

 16.3 

 28.9 



250.7 



1895. 



76.6 

 56 2 

 48.2 

 35.0 

 44.6 

 36.2 

 17.3 

 19.6 

 27.0 



13.5 



14.8 



15.2 



124.6 



270.8 



1896. 



511.5 

 314.6 

 266.0 



182.8 

 138.9 



Numbers enclosed in a parenthesis rest on observations made 

 on a single day. 



The females which have lived over winter produce at least 

 three broods of young, and die in June, chiefly in the early part 

 of the month. Those individuals which have lived over winter 

 are readily distinguished from those hatched in the spring by 

 the smaller size and different shape of the head. It is easy, 

 therefore, to determine the average length of their life at about 

 six to eight months, from early October to early June, as a 

 maximum. It is not possible to get similar data for the sum- 

 mer form of this species, for the shape of the head-crest gradu. 

 ally alters in all individuals as the water cools in the autumn. 



The swarms of young produced in October rapidly diminish 

 in number at first, but an equilibrium is reached by the first of 

 January, and thenceforward the decline through the winter is 

 very slow, or imperceptible. The statements made regarding 

 Diaptomus fully apply to this species also. During April and 



