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, it is 351. In 1896 a much greater proportion of 

 the change in levels took place (Pt.-L, PI. 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., PI. 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., PI. 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 neptunius 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., PI. XL), is greatly exceeded by that in 1896 (2,323), when the 

 hydrographic conditions during summer were much disturbed (Pt. I., 

 PI. X.). 



