TEMPORARY POND COMMUNITIES E73 
vegetation above the water including the birds are about the same as 
in the preceding sub-formations. 
Permanent and temporary swamps are covered with trees. The 
most important permanent swamps are the tamarack swamps. The 
aquatic phase of these will be discussed in connection with the tamarack 
swamp itself (p. 193). Temporary swamps will be discussed under the 
head of temporary forest ponds. ; 
2. TEMPORARY POND OR TEMPORARY SWAMP AND MARSH 
FORMATIONS 
The situations known as temporary ponds, temporary marshes or 
swamps, or summer dry ponds, are common about Chicago and usually 
contain water in early spring, drying before the first of June. At some 
points at the south end of Lake Michigan much sand has been removed 
for commercial purposes and frequently the workmen remove it to points 
below the ground-water level of the spring months and accordingly make 
temporary ponds which have pure white sand bottoms. A few of these 
have been studied, one when it was one year old, another when about 
twelve years old. These were compared with ponds of the horizontal 
series which are much older. 
a) Bare-bottom association—Twelve-months pond association (Sta- 
tion 40; Table XXXVII): In April, 1910, we found this pond full of 
filamentous algae, and containing rotifers, copepods, and ostracods, the 
eggs of all of which will probably withstand drying and may have 
blown into the pond during the preceding dry seasons. There was a 
single full-grown snail (Physa gyrina), a small individual (probably 
Physa heterostropha), and a small long snail, Lymnaea (probably exigua). 
These snails may have been carried into the pond, from other ponds a 
few rods away, on the feet of turtles or frogs. 
Twelve-year-old pond association (Station 40; Table XX XVII): As 
such a pond as we have just described grows older, the algae continue 
and the reed (Juncus balticus) comes in, together with some sedgelike 
plants. In such ponds the number of species is usually greater than 
at an earlier period. 
In addition to the species found in the twelve-months pond, we 
obtained water-beetles, which are, however, not particularly signifi- 
cant because they may occur in rain pools. Cladocera, the flat snails 
(Planorbis sp.), and the nymphs of damsel-flies and dragon-flies are 
also found. The difference between this pond and the preceding one 
is not great. Indeed, it is only when the bottom of the pond becomes 
