106 
an adventitious planktont. The data are too irregular to trace its 
seasonal distribution. 
As a species it is as well defined as any in the genus. It is not in 
our waters connected by intermediate forms with other species. Its 
assignment to D. lobostoma by Schewiakoff (’93) is not in my opinion 
justifiable unless we regard all forms of Dzfflugza as belonging to one | 
species. 
Difflugia fragosa Hempel.—Average number, 25; in 1896 over 
100. This species occurred in every month of the year but Febru- 
ary, though three fifths of the records and the majority of the in- 
dividuals were found between May and October at temperatures 
above 60°. The data are too irregular to trace the seasonal history 
of the organism, but they suffice to suggest the agency of floods at 
all times and of high temperatures during the summer, as factors 
in the occurrence of the species in the plankton. The shell of this 
form is relatively to that of other species rather heavy, and this fact 
combined with the irregularity of its occurrence seems to justify 
the conclusion that it is largely adventitious at all seasons of the 
year. 
The species exhibits a great deal of variation in the development 
of the central spine—Hempel (’99, Fig. 1)—and in the number and 
arrangement of spines in the accessory circlet. The mammillate 
form of the central spine figured by Hempel is not usually present. 
Individuals in which the central spine is but feebly developed seem 
to connect this species with D. varians, recently described by 
Penard (’02). Otherwise, and in our waters, the species is well 
delimited. 
Difflugia globulosa Duj.—Average number. 7,194; in 1897, 
47,329, the larger number in this year being in part due to a remark- 
able pulse of 1,240,000 early in September. This is the most ~ 
abundant of all the rhizopods in our plankton, occurring most 
frequently and in largest numbers. It is found in every month of 
the year, and in 1898 appeared in every collection except four in De- 
cember. With a few exceptions in the autumn of 1898 (Table I.), 
no large development (exceeding 10,000 per m.*) has taken place 
earlier than May or later than September—that is, at temperatures 
below 60°. The occurrences are most continuous and the numbers 
of individuals are largest during the warmer period between the 
months named. ‘The largest pulse, that of 1,240,000 on September 
