PLANKTON STUDIES ON LAKE MENDOTA. II. 
THE CRUSTACEA OF THE PLANKTON FROM JULY, 
1894, TO DECEMBER, 1896. 
E. A. BIRGE, 
Professor of Zoology, University of Wisconsin. 
INTRODUCTION. 
The following paper is a continuation of the work done by 
myself with Messrs. Olson and Harder, in the summer of 1894, 
published in the preceding volume of the Transactions of this 
Academy. (Birge, ’95.) The study carried on in that month 
showed a vertical distribution of the Crustacea so unexpected 
and peculiar that it seemed to me worth while to continue the 
investigation throughout an entire year. A few observations 
were made in the latter part of August, 1894, and on Septem¬ 
ber 18th, regular observations were begun and were continued 
until the close of December, 1896. During the fall of 1894 ob¬ 
servations were taken on 28 days. In 1895 observations were 
taken on 110 days, and on 126 in 1896. The details of the 
number of observations and of the days on which they were 
taken will be found stated in Table A given at the close of 
this paper. During the late spring and summer months as 
many as three observations per week were taken. During the 
winter season, the late fall and early spring, observations were 
necessarily fewer in number, and occasionally a period of two 
weeks would pass without an observation. At this time of the 
year, however, the Crustacea are not varying greatly in num¬ 
ber, so that small error results from these gaps. 
I had intended at first to carry my observations through one 
year only, but as a peculiar annual development of the crusta- 
