427 
(I am informed that Dytiscus marginalis Linn was formerly obtained from 
this pond in quantity for laboratory dissection material.) I have never 
taken a specimen from the pond. What other forms may have been elimi- 
nated by this “drying up,’ I do not know, because I began to study it at 
this period. 
Not only were the conditions during this period of low level very dif- 
ferent from these preceding it. but the conditions after the dry period were 
also very different. 
When the pond began to fill with water in November, ’OS, the decaying 
wmpphibian larve and other organic matter developed conditions favorable 
to the production of an enormous number of flagetlates. This decaying or- 
ganic debris and possibly the flagellates furnished an immense amount of 
feod for some of the crustacea, especially Cypridopsis vidua Brady. The 
alge are eaten by both the amphibian larvee and Simecephalus vetellus. 
The elimination of the former greatly increased the food material of the 
latter. 
The dragon fly nymphs and possibly Corethra larvee feed on on both 
of these crustaceans. Thus the conditions at this period furnished the 
crustacea an enormous food suppty and few enemies. The result was a very 
great development of crustacea. Especially was this true of Crypridopsis 
vidua Brady. Since the winter of 1990S, conditions which I have not been 
able to determine have resulted in the entire elimination of this form. It 
is evident that variations in the level may result in the elimination of a 
species or its abnormal development. 
Temperature.—The seasonal development of different forms as indi- 
eated in the list and table, is probably due directly or indirectly to changes 
in temperature. The temperature in the water of the pond varies from 
27.8° C to O at the surface (ice) and to 1.8° C at the bottom. 
Except for the first few weeks the temperatures were taken with a 
centigrade thermometer graded to 1/5ths. ‘The winter of 1908-1909 was 
fairly open. Ice formed December 2, lasting until January 20. Ice was 
present the latter part of February but there was none after March 3. 
The maximum thickness of ice for this year was 2.5 inches. The winter 
of 1909-1910 was very severe for this latitude. Ice formed December 7 and 
lasted until March 2 and had a maximum thickness of 9 inches on January 
11. During the first winter, the temperature of the water a few inches 
under the ice, varied but slightly from the greatest density temperature. 
