MuUkowski—The Fauna of Lake Mendota. 427 
tinguish, however, between the detached rachion and the con¬ 
tinued rachion. Since the latter are found on steeper, hence 
more exposed, shores, the molar agents will be more violent in 
these places. Hence, by far the larger number of petrophilous 
forms are absent from the continuous rachion. On the other 
hand, the great majority of them are present in the detached 
rachion. Again, Gomphus fraternus, Epicordulia princeps and 
Molanna uniophila appear to prefer the sand heaps just beyond 
the attached rachion, while Amnicola limosa foregathers directly 
on the breaker line in all parts of the lake in early summer. 
3. The Plant Zone. 
In the plant zone we have to deal with a double problem,— 
that of two substrata. The composition of the bottom is to be 
considered, and that of the vegetation growing upon it. Since 
by far the larger portion of the fauna is phytophilous, the treat¬ 
ment may be simplified into that of the plant areas, and of the 
barrens. The plant area of the lake extends from a depth of 
about one meter to the limit of photosynthesis, which in Lake 
Mendota is at about 6 meters. This area is interspersed with 
small barren patches of sand, or rock, while in other places the 
accumulation of plant marl forms large beds to the hindrance of 
the growth of the plant itself. 
The plant zone may be conveniently divided into two divi¬ 
sions according to the habitus of the plants,—upright plants, 
and recumbent plants. 
A. Upright Plants .— These are formed largely by Pota- 
mogeton amplifolius, Najas, and Vallisneria, and more rarely 
by Myriophyllum and Ceratophyllum. Their characteristic is 
that they tend to reach the surface, but except for their floral 
heads no part is protruded. They extend to a depth of about 
5 meters, usually less. 
Physiologically, they offer the following: (a) exposure to 
molar agents near surface, (b) plentiful clinging surfaces, 
(c) a food supply, (d) shelter. On the other hand, since this 
flora is not an emergent one, it offers no foothold for surface 
breathers. 
The fauna must therefore be composed of water-breathers in 
the first place; beyond this dingers, crawlers, and swimmers of 
all types may find food, shelter and protection. It is among the 
