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 raak_ 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. In a 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. 



Difflugiq constricta Ehrbg. Average number, 46. This species 

 occurs irregularly at all seasons of the year without marked prefer- 

 ence for 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. 3 ) 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 w r hich 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 tho.se 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 fi), 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 is plainly 



