256 



where it " appears almost constantly in the plankton." His investi- 

 gations, however, appear to cover only the months of May-August. 

 Maximum numbers appeared in July, and considerable differences 

 were noted in two successive years. 



I find no previous record of the occurrence of Moina nticrura in 

 American waters. 



Pleuroxus denticulatus Birge. Average number, 5. Occurs in 

 small numbers and irregularly during the autumn and spring months 

 during declining or rising temperatures. The earliest autumnal 

 record is November 2, and the latest, December 15; the earliest 

 vernal is March 8, and the latest is May 31. Egg-bearing females 

 appear in the earlier occurrences in each season. It is evidently 

 adventitious. 



Pleuroxus hamatus Birge was found once March 29, 1898. 



Scapholeberis mucronata O. F. Mull, was recorded in small num- 

 bers in May and August-December through the seasonal range of 

 temperatures. It is apparently adventitious in channel plankton, 

 though not attending flood invasions. 



Sida crystallina O. F. Mull, is rare in the summer plankton. 



Simocephalus serrulatus Koch. Average number, 261. This 

 species appears irregularly in the plankton, generally in small 

 numbers and in isolated occurrences. An exception to this is found 

 in May- June, 1898 (Table I.), when it is found continuously May 

 10- June 14 in numbers which furnish 61 per cent, of the total for 

 all years. There is a slight preponderance of occurrences in May 

 and September, 12 of the 26 recorded appearing in these months. 

 Their irregular appearance in the plankton in general suggests that 

 they are adventitious from the littoral area, especially at times 

 of their maximum development there. The period of their occur- 

 rence in the channel plankton in 1898 was one of rising water, 10 to 

 14 feet above low-water mark a stage permitting free communica- 

 tion between the channel and large areas of slightly submerged 

 bottom-lands. 



Simocephalus vetulus O. F. Mull, appeared irregularly and in 

 small numbers in the plankton in April- June (4 occurrences) and 

 September-December (5 occurrences). It is evidently adventitious 

 in the plankton, coming from the littoral area, though not confined 

 to flood waters. 



