TES 
forms their margin. Chitinous, brownish, or more or less trans- 
parent shells are abundant when pulses occur. Forms which 
connect this species with D. globulosa have been observed. In- 
cluded with )). lobostoma are forms which have since been described 
by Penard (02) as D. gramen, D. gramen var. achlora, and D. 
lithoplites, though I have not found in the Illinois plankton any of 
the last-named with the peculiar tipped horns found by Penard 
upon many individuals of his species. 
Difflugia pristis Penard (?).—A small Dzfflugia was found occa- 
sionally in the filter-paper collections in the colder months, but 
only from November to March. It was often dark, or even blackish, 
resembling in this respect Penard’s D. pristis. Individuals not 
thus darkened approach more nearly D. jallax Penard and D. pulex 
Penard. 
Difflugia pyriformis Perty.—Average number, 368. This species 
occurred in every month except January, but generally in small 
numbers and irregularly. The largest number taken—12,000, on 
May 25, 1896—came with the flood at that time (Pt. I., Pl. X.), and 
all the large occurrences of 1898 came with rapidly rising water 
(cf. Table I. and Pt. I., Pl. XII.). There are no indications of pulses 
during stable conditions, and we must conclude that the species is 
purely adventitious in our plankton. It is one of the largest species 
with a heavy shell, and its flotation is impeded thereby. 
This species is exceedingly variable. The following varieties or 
variants, given specific rank by some writers, have been noted, and 
are included with D. pyriformts in the enumeration: D. pyriformis 
var. nodosa Leidy, D. pyriformis var. claviformis.Penard, D. pyriformis 
var. venusta Penard, and D. pyrtformis var. lacustris Penard. A 
more slender and smoothly contoured form than the last is not 
uncommon. 
D. capreolata Penard and D. bacillifera Penard were also found, 
but are rare. 
Difflugia rubescens Penard was taken but once—on May 25, 1896. 
Difflugia tuberculosa Hempel was also found but once in the 
planktons enumerated, though Hempel (’99) reports it as appearing 
occasionally from August to November in 1895. 
Difflugia urceolata Carter was taken only in April and May, 1896, 
in small numbers at temperatures of 66°—80°. 
