25% 
OSTRACODA. 
The species of this order are in the main, during adult life, 
limicolous forms found in the littoral or bottom ooze or amid the 
decaying organic matter which accumulates in these regions. The 
current, the movements of fish and other large aquatic organisms, 
the action of waves along shore and in shoal regions, all tend to bring 
these animals into the limnetic fauna. Their centers of distribution 
are thus in littoral or bottom regions, and in the adult stage they 
are almost wholly adventitious in the plankton of our waters. In 
1898 the average number per m.* was 191, but in 1897, a more stable 
year, only 97. 
The seasonal distribution of their occurrences in the plankton 
indicates a decided predominance in March—October, in which 
months all but 6 of the 73 records were made. In these months 
from 23 to 82 per cent. of the collections contained Ostracoda, while 
in December—February only 8 to 20 per cent. The percentages in 
April-September are all above 45 per cent., and the numbers per 
m.* are also larger in this period (see Table I.) The tendency 
toward a vernal increase is apparent in the records of each year in 
much the form in which it occurs in 1898 (Table I.). The numbers 
are always small at all seasons, not exceeding 1,600 per m.* even 
in the vernal season. 
The seasonal distribution is such that the greater part of the 
occurrences and the greater number of individuals appear in the 
plankton during the warm season, that is, above 50°. Thus, in 1898 
all but 4 of the 24 occurrences and 99.5 per cent. of the indivi- 
duals appear after the vernal rise passes 50° and before the 
autumnal decline reaches that point. The Ostracoda are plank- 
tonts of the warmer season. 
It is significant that the Ostracoda in our plankton collections 
are largely young or immature individuals. In 1898, for example, 
74 per cent. of individuals observed were not adult, and most of 
these appeared in April-June. Their occurrence in the plankton 
can not be traced to the action of flood waters. It thus seems 
probable that the young Ostracoda may temporarily adopt more 
of a limnetic habit than the adults. 
No attempt was made to systematically identify the Ostracoda of 
the plankton catches. The list of species and the notes thereon 
