140 POND COMMUNITIES 
A decrease in depth, due to the accumulation of humus and the lowering 
of the ground-water level, is to be noted in the older ponds. The series 
is, then, an ecological age-series, and throughout our discussion we refer 
to earlier and later phases of the various associations concerned. 
III. ComMUNITIES OF PONDS 
I. THE PELAGIC FORMATION 
We have in the ponds a pelagic formation. Though it is limited 
in number of species, many of which breed on the bottom, it is similar 
to that of larger lakes. We have found little difference in the pelagic 
species inhabiting younger and older permanent ponds. Diaptomus 
reighardi has not been taken from ponds filled with the vegetation 
which reaches the surface. Other species are about the same in 
the different permanent ponds. The pelagic formation is poorly 
developed. 
2. PIONEER FORMATION (TERRIGENOUS BOTTOM) 
(Ponds, 1, 5, 7) (113) (Stations 9 and 32; Tables XXVII and XXXIV) 
The youngest ponds of the Chicago area are near Waukegan. The 
outer end of the Dead River receives the force of the winter waves from 
the lake and the bottom is bare, with a few scattered aquatic plants. 
Here animals are few. We have taken only a few invertebrates. The 
fish present probably get their food from the older parts farther back 
from the lake. The fish are: the pike (Esox lucius) which prefers clear, 
clean, cool water (79); the red-horse (Moxostoma aureolum) which dies 
in the aquarium if the water is the least bit impure, and which also suc- 
cumbs to any impurities in its natural environment (79); Notropis 
cayuga, which prefers clear waters; the common shiner (NVotropis 
cornutus) which breeds on bare bottom (105), and the white crappie 
(Pomoxis annularis) which lives in streams. On the bottom at such a 
period one is likely to find the larvae of caddis-flies (Goera sp.), snails, 
mussels, etc., but we have found none in the Dead River. 
Vegetation quickly captures parts of such a pond. Chara is the 
first plant to cover parts of the bottom. After this has happened, the 
pioneer formation may still continue. In Pond 1 of the series of special 
study (Fig. 85) we have a considerable area of bare sand, and the forms 
present are the caddis-worm (Goera sp.) and the mussels (Anodonta 
marginata and grandis, and Lampsilis luteola). These are preyed upon 
by muskrats (Fig. 86). There are a number of fish that belong to this 
