Muttkowski—The Fauna of Lake Mendota. 423 
Before passing on, note must be made as to the extent of the 
fauna of the rock shore. Vertically, the fauna extends down¬ 
ward below the breaker line, often for several meters. Thus, 
Psephenus lecontei, Ephemera varia, Heptagenia, Ecdyurus, 
Stenelmis crenatus, Chironomus digitatus, C. lobiferus, etc., are 
found not only occasionally, but in fact quite frequently on 
rock bottoms at depths exceeding two, three, and even six meters. 
This is an important phenomenon. These same species are also 
found under the rocks of milder rapids, in creeks, rivers, and 
falls. If we note in addition that several of the species have 
been found on the rocky reefs in the middle of the lake, it must 
be evident at once, that current can hardly be considered the 
primary factor in the distribution of. these forms; that on the 
contrary it is probably the condition of the substratum,—namely 
the rock bottom,—which is the determining factor, and that cur¬ 
rent and the ability to respire under difficult conditions are 
secondary factors. 
B. The Cohhle (Gravel) Shores .—Cobble shores occupy 
over one half of the shore lines of Lake Mendota (inclusive of 
bars). Their physical character is that of stones of varying 
size intermixed with gravel and coarse sand, all worn to smooth¬ 
ness by molar agents. Hence they are analogous in their con¬ 
ditions to those offered by the rock shores, and the difference is 
a relative one, expressed mainly by the facts that there are 
fewer and smaller and also smoother rocks, that there is an 
abundance of pebbles and of coarse sand,—in general, that there 
are fewer clinging surfaces. 
Physiologically, the same conditions hold true as for the rocky 
shores. Yet the finer composition of the substratum permits 
a comparably greater activity of the molar agents upon it; in 
heavy weather there is a steady oscillation of sand and gravel 
carried by waves and undeiTow. And this fluctuation of the 
rubble may be said to accentuate the need of protective shelter 
for this association. 
Thus, soft-bodied animals,—such as Planarians, various Oli- 
gochaeta, Ceratopogonine larvae, the larvae of may-flies,—and 
adult bettles are much rarer in the stony association than among 
the rocks. However, it is necessary to distinguish between ab¬ 
sence due to molar agents and absence due to food supply. The 
alga Cladophora, so conspicuous on the rocky shores, is found 
in more isolated patches; like soft-bodied Planaria, it cannot 
