102 



average number of individuals in these cases was, however, only 

 230 per cubic meter as against 1,443 when the temperature was 

 above 55, or, if below, when floods prevailed. The seasonal and 

 numerical distribution of occurrences and individuals alike point 

 to the agency of floods and higher temperatures in the introduction 

 of Arcella into the plankton from its usual habitat, the bottom and 

 the shore. 



This species occurred in water ranging in temperature from 32 

 to 89. Being a bottom form, the plankton data do not afford a 

 satisfactory basis for determining its true seasonal distribution and 

 optimum temperature. The maximum number found, 25,272, was 

 in water at 32; but this was an isolated occurrence in a flood, and 

 serves only to illustrate the irregularity of distribution in the 

 plankton of tycholimnetic organisms. 



Centropyxis aculeaca Stein. Average number, 570. This species 

 has appeared in collections in every month of the year, but its 

 sequence is frequently interrupted and its numbers are quite irregu- 

 lar. Practically without exception all the larger occurrences attend 

 rising flood waters. It is evidently adventitious at all seasons of 

 the year. 



Centropyxis aculeata var. ecornis (Ehrbg.) Leidy. Average 

 number, 604. In former years this species was less frequent than 

 the preceding species. Its appearances in the plankton tend to 

 coincide with those of C. aculeata (Table I.), and are doubtless due 

 to the same causes. Thus in the February flood of 1898 there is a 

 pulse of 12,636 of C. aculeata and one of 9,477 of var. ecornis. 

 C. l&vigata Penard seems to be identical with this variety. The 

 data concerning both C. aculeata and its variety ecornis are too 

 irregular to throw any light on the seasonal cycle of these adventi- 

 tious planktonts. 



Cochliopodium bi limbo sum (Auerbach) Leidy. Average number, 

 1,384. This species was found in the plankton during 1898 in 

 irregular numbers in 27 of the 52 collections. The distribution of 

 the occurrences affoids indubitable proof of their close dependence 

 upon flood waters. In 15 of the 27 cases Cochliopodium appeared 

 with a rising river, and in all but 6 cases, in periods of considerable 

 movement in river levels (cf. Table I. with PL XII., Pt. I.), such 

 as the rising flood of January and February and the repeated minor 



