392 
Birge—The Crustacea, of the Plankton. 
Each species of Crustacea, also, has individual peculiarities of 
distribution, which recur from year to year with surprising 
similarity and which are independent of the absolute number 
present. These peculiarities appear when the average of any 
species is taken, although of course it is entirely possible that 
the distribution should depart widely from this average at any 
single observation. In general it may be said that the summer 
distribution of the Crustacea follows very closely the figures 
which are given in my former paper (Birge, ’95), and that the 
variations in the distribution which have been found during the 
two years and a half succeeding the observations reported in 
that paper, have been of the same type and in general of the 
same degree as those which were found during the single month 
of our first study. It seems to me, therefore, unnecessary to 
point out again these variations in detail for each species. 
In order to show the resemblances and differences in the per¬ 
centile distribution of the Crustacea during the summer months, 
when their numbers are great and the distribution is most 
characteristic, I have averaged this distribution for the 
summers of three years: 1894, 1895, 1896. I have included the 
three standard representatives of the limnetic Crustacea which 
are regularly present in full numbers during this time; Diap- 
tomus, Cyclops, D. hyalina. The period included is from the mid¬ 
dle of June to the middle of September, in 1895 and 1896; and 
July and August of 1894. It will be remembered that no ob¬ 
servations were taken in 1894 before July or during the first 
part of September, but as the summer conditions were thor¬ 
oughly established at the first of July of that year and contin¬ 
ued until the first of October no noteworthy difference would 
appear in the averages had it been possible to extend the period. 
It will be seen from these averages that the distribution of Cy¬ 
clops in the three years in question varies surprisingly little; 
the percentile difference in the 0-3 m. level being less than 
1.5. This close correspondence in distribution exists in spite 
of the fact that the numbers of the genus were very different 
in the three years. The same general agreement is seen in 
the tables of semi-monthly distribution. Compare July, 1894 
and 1896 in Table C, Appendix. 
