Muttkowski—The Fauna of Lake Mendota. 
429 
ered with a thick encrustation of an alga. This encrustation 
gives shelter and food to a considerable number of micro-organ¬ 
isms, so that it can offer some attraction to the fauna of the lake. 
Hyalella azteca, Psephenus lecontei and several ubiquitous 
Chironomids are the only members of the macrofauna that I have 
found on the reefs. Psephenus lecontei was the biggest sur¬ 
prise of all. This is a rock shore species, which pupates, co¬ 
pulates, and oviposits on land. How it gets to the reefs,.how it 
reaches the land to pupate is indeed a mystery. It is evident, 
however, that the condition of the substratum is of decisive in¬ 
fluence upon Psephenus, for it occurs on most of the reefs, even 
at 8 meters depth. 
The Dysphytal (Sublittoral) Area. 
Physiographically, this area lies immediately below the plant 
area, and marks the limit of photosynthesis. It is characterized 
by a heaping of slowly decaying plant and other organic mat¬ 
ter. It may lie above, or below, the wave limit. In Lake Men¬ 
dota it happens to coincide with the limit of wave-action (wave- 
base) and is specifically characterized by the great amount of 
decomposed shell,—a shell zone. In some of the clear lakes 
of the north the euphytal or photosynthetic region may extend 
considerably below the normal wave-base and the shell zone 
then lies within the euphytal area. 
The Shell Zone .—As just stated, in Lake Mendota the shell 
zone is physiographically coincident with lower wave action, 
and is marked physically by the great accumulation of shell, 
with an admixture of mud and sand. The heaviei particles 
carried by the water have been deposited during their trans¬ 
portation from the shore and only the lighter fragments of 
shell are swept farther downward, where they accumulate by 
their own weight. Beyond this point only mud is carried. 
The shell zone is also marked, in Lake Mendota, by the or¬ 
ganic offal from the plant area above. Many of the plants, 
though uprooted, will be able to maintain themselves for a 
while, but will finally succumb to the lack of sufficient continued 
light for photosynthesis. 
Hence the following conditions are offered to the fauna: (a) 
absence of molar agents, (b) absence of clinging surfaces, (c) 
absence of living plant food, (d) organic waste, i. e., organic 
