104 
importance in its economy. It includes a number of forms notorious — 
for their variability and for the difficulty with which specific dis- 
tinctions can be applied. I shall discuss the species as they were 
enumerated, and shall correlate my work with Penard’s (’02) recent 
elaborate analysis of the species so far as I can with the aid of my 
notes in the absence of the collections. Opinion as to the validity 
of the species is expressly withheld excepting in those instances in 
which it is formally stated. 
Difflugia acuminata Ehrbg.—Average number, 315. This spe- 
cies has occurred in every month of the year and in 83 out of 180 
collections. In 1898, two thirds of the occurrences and three fourths 
of the individuals were taken between May 1 and October 30, at 
temperatures above 70°. In this year there are six recurrent pulses 
from June to November, but all but one of these are found on rapidly 
rising flood waters, and they bear no constant relation to the pulses 
of diatoms previously noted, with which in some instances they 
are intercalated, though this is not regular or constant. Similar 
tendencies to appear with floods and in greater numbers and more 
frequently in summer can be detected in records of other years. It 
was more than twice as abundant in 1896—a year of interrupted 
hydrograph (Pt..I., Pl. X.)—as in 1898. This is one of the larger 
and heavier rhizopods, and its occurrence in the plankton 1s doubt- 
less adventitious, due to floods and currents, and its greater numbers 
and frequency in the summer may result from its greater abundance 
at that season in its natural habitat, the shore and bottom, and 
perhaps, also, from its lighter specific gravity during the warmer 
season. An illustration of this appears on the rising flood of June, 
1897, when the maximum number recorded (10,000 per m.*) oc- 
curred. 
The shell of this species is exceedingly variable in size, constitu- 
ent particles, and proportions. A number of forms separated by 
Penard (’02) and others as distinct species were grouped under D. 
acuminata in the enumeration. The greater number of these belong 
to the type designated by this name by Penard (02). D. acuminata 
var. inflata Penard and the somewhat similar D. elegans Penard are 
not uncommon. JD. acuminata var. umbilicata Penard, D. elegans 
var. teres Penard, D. curvicaulis Penard, D. lanceolata Penard, and 
D. scalpellum Penard occur also, but are rare. 

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