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 
thou sands per square meter. 
1894. 
1895. 
1898. 
October 16-31. 
252.5 
76.6 
511.5 
November 1-15.. 
183.1 
56 2 
314.6 
November 15-30. 
48.2 
268.0 
December 1-15. 
121.5 
35.0 
182.8 
December 16-31 .. 
(49.0) 
44.6 
138.9 
January 1-15... 
40.8 
36.2 
January 16-31. 
(55.9) 
17.3 
F'ehrnary 1-11 . 
(75.3) 
19.6 
TPehrnary 15-23. 
65.8 
27.0 
March 1-15. 
34.7 
March 16-31. 
63.6 
13.5 
April 1-15 . . 
26.4 
14.6 
April 16-30. 
16.3 
15.2 
May 1-15 . 
28.9 
124.6 
May 16-31... .. 
250.7 
270.8 
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 
