Methods of Counting and Computing the Crustacea. 435» 

 August 1, 1894. 



1 



6. P. M. j 



9. P 



. M. 



Diaptomus. 



No. 



Per cent. 



No. 



Per cent,. 





1788 



41.04 



1572 



59 21' 



3-6 



1068 



24 51 



462 



17.40* 





1110 



25.48 



486 



18.31 





384 



8.81 



105 



3.95 



12-15 .. 



3 

 4 



.07 

 .09 



19 

 11 



.72 





.41 





4357 





2655 





After computing the percentage for each series of observa- 

 tions, the average distribution of each of the species was de- 

 termined for each period and for the entire month. In deter- 

 mining these averages the total number of Crustacea obtained) 

 from each level was found and divided by the number of obser- 

 vations. The sum of all these results form the base on which; 

 the average percentages were computed. In this way observa- 

 tions could be used which did not form part of complete series^. 

 and in several cases one or two more observations were included 

 in the upper level than in some of those below. The number of 

 Crustacea, however, was in general so nearly uniform that the 

 inclusion or omission of these partial observations made no ap- 

 preciable difference in the percentages. 



In determining the averages for the month a similar method 

 was used. Each series of observations was used by itself, and 

 the average for the month is the average for the observations r 

 and not the average of the different periods taken as units. 

 Since these periods are unequal in length they have a different 

 value in determining the total average, but it seemed to us that a 

 fairer average would be reached by taking the observations in 

 this way, rather than by considering each period as a unit. 



Our discussion of results is based rather on percentages of 

 distribution than on actual numbers of Crustacea per square or- 

 cubic meter. This has been done for two reasons : First, the 

 coefficient of the dredge was not determined at the time that 

 the computations were made, and it was therefore impossible for- 

 29 



