* FAUNA OF A SMALL BROOK 365 
the nature of the physical and vegetational features of the en- 
vironment. For example the rich fauna of many of the larger 
pools consists of a surface film association, a bottom association, 
a free-swimming association and a marginal or transition zone 
association, etc. For convenience it has been deemed advisable to 
consider the life of this brook under two main heads, (1) the 
upper brook, and (2) the lower brook, with various subdivisions 
into associations where the data permit. 
(1) The upper brook. 
The upper part of the stream, as has been mentioned already, 
consists of a slowly moving shallow stream of water, with occa- 
sional clear cut channels, but for the most part creeping through 
semiboggy, grassy areas. The bottom was muddy throughout, 
covered with algae, and containing much decaying vegetable 
matter from last year’s foliage. 
The animal life here consisted of various Entomostracha, 
Planaria on the bottom, with Tipulid larvae as the prevalent 
larger form. Other undetermined dipterous larvae were taken. 
A few crayfish burrows were observed in the wet ground. 
(2) The lower brook. 
Here the cutting of the stream through heretogenous material 
has developed well defined rapids and pools, each deserving of 
separate consideration. 
THE RAPIDS. 
The rapids of this stream are peculiar in that they consist al- 
most wholly of stratified rocks over which the water drops, thus 
affording scant shelter and places of attachment for the more 
common forms inhabiting rapid water (Plate XL, figure 2). 
These stones were covered with algae and upon each shelf lay 
masses of decaying leaves. 
No animal form was strictly peculiar to these rapids. The 
artificial burrows of Chironomous larvae occurred here in num- 
bers, many with the larvae inside. Other forms such as Tipulid 
larvae, Stratiomys larvae and Hydrophilid beetles were taken 
from among the decaying leaves and from the crevices in the 
rocks. These were also found in the pools. 
