142 
within these limits excepting a single record December 29, 1896, at 
35°. The lower temperature limits are thus near 45°, and the num- 
bers are all small below 60°. The occurrences are never in very large 
numbers, and significant pulses do not appear—an indication that 
the species is adventitious in the plankton. The relative numbers in 
different years is suggestive. In 1896, with a total movement in 
river levels of 45.7 feet, the average number per collection is 770; in 
1897, with a total movement of 44.8 feet, the number is 271; and in 
1898, with 67.2 feet, itis 351. In 1896 a much greater proportion of 
the change in levels took place (Pt. I., Pl. XI.) during the summer, 
when P. megalotrocha is present. With this in mind, it is apparent 
that a disturbed hydrograph tends to increase the number of this 
species in the plankton. A comparison of the individual occurrences 
(Table I.) with contemporaneous conditions of the hydrograph 
(Pt. I., Pl. XII.) in 1898, and in previous years also, shows that most 
of the larger records were made in planktons from a rising river. 
For example, the largest record made—8,000 on September 27, 1898 
—is on the crest of a slight rise (Pt. I., Pl. XII.). Some, however, 
appear in stable conditions, and may be attributed to the other causes 
of disturbance of the bottom and littoral fauna which tend to bring 
its constituents temporarily into the domain of the plankton. 
Rotifer neptumus Ehrbg.—Average number, 425. This species 
was found in the plankton in every month of the year but February, 
and thus throughout the whole temperature range. Between Novem- 
ber and March the records are scattered and the numbers small, 
while it is continuously present in larger numbers from March (50°) 
till late in October (50°-60°). The optimum temperatures thus seem 
to lie above 50° in our waters. The largest numbers recorded (22,224, 
April 29, 1896, at 72°, and 6,400, May 17, 1898, at 64°) attend the 
vernal volumetric pulse. Aside from this season, well-defined and 
symmetrical pulses are rarely traceable in the small numbers recorded. 
Some of the larger records, for example that of July 28, 1896 (10,200), 
attend rapidly rising water, but dependence generally upon this 
agency for presence in the plankton is less directly evident in this 
species than in the preceding. As also in-the case of R. tardus, the 
average number (246) in 1897, a year of more stable hydrograph 
(Pt. I., Pl. XI_), is greatly exceeded by that in 1896 (2,323), when the 
hydrographic conditions during summer were much disturbed(Pt. I., 
Pile Xe): 
