368 



Birge — The Crustacea of the Plankton. 



either of all the Crustacea or of single species. It will be 

 seen from the tables giving the maximum and minimum catches 

 for each two week period that in the more numerous species the 

 maximum catch is about four times the minimum, when the 

 species is neither increasing or decreasing in numbers to any- 

 marked degree. Where the species is present in small numbers, 

 the range of variation is far greater. Thus, in July, 1895, Lep- 

 todora showed a variation from 1 to 50 individuals in the 39 

 catches made during that month. It varied from 1 to 19 in 

 catches made on the same day, and was wholly irregular in its 

 variations during the month. During the same month the catch 

 of Cyclops varied from 1290 to 6100; and on no day were two 

 catches made in which one was double the other. In each of 12 

 days in 1895 and 1896 two catches were made at points about 

 two kilometers apart, and the ratio of the predominant species 

 in these 12 cases was as follows: 



Diaptomus 

 Cyclops ... 

 D. hyalina 



Average 

 ratio. 



A:B::1:1.62 

 A:B::1:1.55 



A:B::1:1.58 



Maximum 

 ratio. 



1:2.4 

 1:2 

 1:2 



Minimum 

 ratio. 



1:1.1 

 1:1.1 

 1:1.1 



In each case A denotes the smaller catch, which was about 

 equally divided between the two stations. 



Again, if comparisons are made of catches extending over 

 a period of time when the average number remains nearly con- 

 stant, and when there is no reproduction, the distribution can 

 readily be inferred. Fifty -six catches of Diaptomus were made 

 between December 1st, 1894, and March 30th, 1895. Of these 

 there were : 



Below 10,000 per square meter, 1 catch. 

 Between 10,000 and 20,000, 14 catches. 

 Between 20,000 and 30,000, 21 catches. 

 Between 30,000 and 40,000, 12 catches. 



Between 40,000 and 50, 000, 5 catches . 

 Between 50,000 and 60,000, 2 catches. 

 Over 70, 000 per square meter, 1 catch. 



The figures also show that all of the December and January 

 catches were below 30,000, all of March above 20,000, and only 

 about one-fourth of them below 30,000; while the February 



