224 
Hydrographic changes affect the Cladocera by increasing the 
amount of silt and flocculent debris in suspension, which, by ad- 
herence to the swimming antennz and flotation processes of the 
animal, tend to impede its movements and sink it to the bottom, 
where it is removed from its normal feeding area and readily becomes 
the prey of the larger organisms of the bottom fauna. Barren flood 
waters also tend to displace and wash away in the increased current 
the Cladocera which have developed in the stream, and to afford 
both less food and less time for their further development. 
The occurrences of the total Cladocera fall into the type of 
recurrent pulses, though with slightly less distinctness than in the 
case of individual species of the group. Such pulses can be traced 
in all seasons in which records were made at short intervals, and 
suggestions of their occurrence appear in the less frequent records 
of other seasons. Thus in July-December, 1897, (Pl. IV.), there 
are 6 well-defined pulses culminating at intervals of 3(1), 4(2), 5(1), 
and 6(1) weeks. In 1898 (Table I.) the pulses are less regular in 
the flood waters of the disturbed year. In 1896, when records were 
frequent, we can trace pulses in March, May, June, July, August, and 
September. The character of these pulses is well illustrated in the 
vernal pulse of 1898 (Table I. and Pl. IV.), culminating June 7 at 
136,000. The species which share in this pulse are Alona affimts, 
A. costata, A. quadrangularis, Bosmina longirostris*, Cerrodaphnia 
scitula*, Chydorus sphericus*, Daphnia hyalina*, D. cucullata*, 
Diaphanosoma brachyurum, Leptodora hyalina, Macrothrix laticornts, 
Moina micrura, Pleuroxus denticulatus, Scapholeberts mucronata, and 
Simocephalus serrulatus. Of these, only the five marked by the 
asterisk occur in numbers sufficient by our methods to delineate a 
pulse. The other species are accordingly of little consequence in 
modifying the form or location of the pulse. The June volumetric 
pulse: (Part I., Pl. XIl-) culminates [une 14 at 6.99 cm" pes aise 
though the record for June 7 is also high (5.28). The cladoceran 
pulse culminates June 7 at 136,000. On this same day four of the 
dominant species also reach their culmination, viz.: Bosmina 
longtrostris (62,800), Certodaphnia scitula (55,800), Daphnia cucul- 
lata (3,400), and D. hyalina (11,600), the remaining 2,400 being 
contributed by other species. Chydorus sphericus, which appears 
this spring only in small numbers, attains its maximum (7,880) on 
May 24, two weeks earlier, though the record for May 31 is also high 
