105 
Difflugia bicuspidata Rhumbler.—Average number, 76. A sep- 
arate record was kept of this bicuspid type in the later years of 
our collections. Penard (’02) regards it as a synonym of his D. 
elegans, though it would seem to be as worthy of specific distinction 
as many other variants to which he accords this rank. It varies 
greatly in the relative development of the accessory “horn,” 
which is sometimes but a mere elevation near the base of the main 
horn. Individuals with equal and symmetrical horns represent the 
other extreme. Ina few cases tricuspid individuals have been seen, 
evidencing a tendency to vary towards the type found in D. varians 
Penard and D. fragosa Hempel. 
This form was about one fourth as abundant as D. acuminata, 
and eight of the ten occurrences fall between May and October, usu- 
ally with D. acuminata and presumably for the same reasons. 
Difflugia constricta Ehrbg.—Average number, 46. This species 
occurs irregularly at all seasons of the year without marked prefer- 
ence tor the warmer months, and often, but not always, with flood 
waters. It occurs throughout the whole range of temperatures, and 
the largest number (2,778 per m.*) appeared during the decline of the 
spring flood. Data are too infrequent to establish any seasonal 
routine. 
This species varies greatly, and is connected by an unbroken 
series of variants with the genus Centropyxis. Penard (’02) also 
notes the existence of this connection, and states that after careful 
search he was unable to find any constant distinction which would 
suffice for its separation. In my enumeration only the elongated 
and smooth individuals were referred to this species. The spinose 
forms were referred to Centropyxis aculeata, and those similar in 
form to the spinose type; but those free from spines, to C. aculeata 
var. ecornis. 
Difflugia corona Wallich.—Average number, 36. In 1896, when 
the hydrograph was much disturbed, the average number was more 
than twice as great. This superb species was found in every month 
of the year except December, but never in large numbers. Its 
large size (200-300 «), and its heavy shell militate against its pres- 
ence in the plankton, and its occurrences are irregular and its num- 
bers few. There is no marked preference for warmer months, and 
four fifths of its occurrences are in rising flood waters. It 1s plainly 
