276 Birge — The Crustacea of the Plankton. 



these results in a graphic form so that they might appeal to the 

 eye, and to print only the summaries of my observations ; rather 

 than to confuse the reader by presenting him with the great 

 mass of figures which would be needed to exhibit the results of 

 the single observations. 



In preparing the diagrams which accompany this paper, the 

 average number of Crustacea for each two-week period was 

 determined and was platted at the center of the space rep- 

 resenting the period; and the averages of successive periods 

 connected by a line. 



It has been found impossible to use the same scale in platting 

 the annual distribution of the different species of the Crustacea. 

 Where numbers range from less than 25,000 to over 3,000,000 

 per square meter, it is not practicable to use the same scale for 

 all species. The scales employed range from 25,000 to one ver- 

 tical space, to 200,000 for the same distance. In all cases the 

 scale is stated on the margin of the diagram. No attempt is 

 made to show by a curve the rate of variation within the two- 

 week period, since this variation is quite too irregular to per- 

 mit a curve to be drawn with any accuracy. 



I had intended to introduce this paper by a preliminary account 

 of lake Mendota accompanied by a hydrographic map. Some 

 hundreds of soundings have been made by myself and by the De- 

 partment of Civil Engineering of the University of Wisconsin, 

 but the preparation of the map has been delayed, and it is there- 

 fore impossible to insert the account at this place. I must 

 therefore refer to the brief account given in my former paper, 

 merely stating here that the lake is about 6 miles (9 kilometers) 

 in length by 4 miles (6 kilometers) in greatest breadth, of 

 a somewhat regular shape. No greater depth than 24 meters 

 has been found; a large part of the lake is deeper than 18 me- 

 ters, and the bottom is very flat without irregular depressions. 

 The principal observing station was near the southern side of 

 the lake, about 2,700 feet (850 meters, from t^e southern shore, 

 and in 18.5 meters of water. The second principal station was 

 about a mile and a half (2 kilometers) from the southern shore, 

 and in 22 meters. The principal station was marked by a buoy, 

 so that the observations were taken at the same spot. 



